"We need guarantees," he said in Brussels as 27 EU leaders backed the bloc's Brexit negotiating guidelines.
The rights of EU citizens to live, work and study in the UK is one of three topics they want dealt with in the first phase of Brexit talks.
Negotiations will start after the UK election on 8 June.
Mr Tusk put citizens' rights centre stage at a news conference after EU leaders - minus UK PM Theresa May - nodded through the guidelines in a matter of minutes.
"Over the past weeks, we have repeatedly heard from our British friends, also during my visit in London, that they are ready to agree on this issue quickly," he said.
"But I would like to state very clearly that we need real guarantees for our people to live, work and study in the UK.
"The same goes for the Brits," living on the European continent, he continued.
UK citizens living in EU countries and non-UK EU citizens living in Britain are estimated at 4.5 million.
The guidelines: Key points
- "Divorce" settlement - first phase of talks dealing with existing UK financial commitments to the EU, Northern Ireland border, residence rights of EU citizens
- UK trade agreement to be discussed only when first phase of talks reaches "significant progress"
- Unity in negotiations - individual EU members won't negotiate separately with UK
- No cherry picking from bits of the single market
The EU's negotiating guidelines, first proposed by Mr Tusk in March, list citizens' residency rights, settling Britain's financial commitments to the EU and avoiding a "hard" border between the Irish Republic and Northern Ireland as the three top issues needing agreement in what are termed "separation talks".
Only once "sufficient progress" is made on these topics can talks touch on the UK's future relationship, including any trade deal, with the EU.
The UK government, however, has pushed for parallel negotiations on trade.
Speaking after the summit, European Commission President Jean-Claude Juncker again stressed that separation talks could not run in parallel with talks on a future trade deal with the UK, backing the line taken by German Chancellor Angela Merkel as she arrived in Brussels.
EU officials said leaders burst into applause as the negotiating stance was waved through at the summit.
EU leaders and officials were keen to stress the EU's unified position on Brexit. Chief negotiator, Michel Barnier, said: "We are ready... we are together."
The fact that the guidelines approved today haven't changed all that much in the past month shows that the EU's claim to have a unified position on Brexit is more than skin deep.
The other 27 countries do see a common purpose in sticking together; and if anything the main changes in language - on a single financial settlement and on the rights of EU citizens in the UK - toughen up the conditions that the UK will have to meet.
Of course there are differences of emphasis in different national capitals - Poland is understandably more concerned than most about the rights of its citizens in the UK because there are so many of them; the Dutch are eager to start talks on future trade relations with the UK sooner rather than later, but they also want to ensure that the UK pays its divorce bill in full.
For now the emphasis on unity is real, and the determination for the EU to negotiate as one should not be underestimated in London.
Speaking earlier, French President François Hollande said there would inevitably be "a price and a cost for the UK - it's the choice that was made".
"We must not be punitive, but at the same time it's clear that Europe knows how to defend its interests, and that Britain will have a weaker position in the future outside Europe, than it has today within Europe."
On the issue of the UK's financial obligations, EU officials estimate that Britain faces a bill of €60bn (£51bn; $65bn) because of EU budget rules. UK politicians have said the government will not pay a sum of that size.
Britain certainly won't tamely accept that it has to pay a huge divorce bill - but it's likely to find the Europeans united on the concept if not the precise amount, the BBC's Kevin Connolly in Brussels says.
UK Brexit Secretary David Davis said in response that both sides wanted the negotiations to be conducted with goodwill.
But he added: "There is no doubt that these negotiations are the most complex the UK has faced in our lifetimes. They will be tough and, at times even confrontational".
|Donald Tusk wants a serious response from the UK. AFP|
EU leaders agree on tough stance at special Brexit summit
EU leaders have unanimously agreed tough negotiating guidelines for Brexit talks with the UK, suggesting they will demand that Britain agrees on payments to the bloc before considering a new trade deal.
The heads of the remaining 27 EU countries agreed to adopt the draft guidelines issued by Donald Tusk last month less than 15 minutes into a special summit in Brussels on Saturday.
The European council president tweeted that a “firm and fair political mandate” for the negotiations was now ready. A senior EU source said the leaders’ decision took only one minute of discussion.
When the formal negotiations between the EU and the UK begin in June, the British government will be told it needs to resolve the key divorce issues of citizens’ rights, the estimated €60bn (£51bn) divorce bill and the Irish border before any talks on a future trade deal can begin.
As he arrived at the summit earlier in the day, the French president, François Hollande, said: “There will inevitably be a price and a cost for Britain, it’s the choice they made.
“We must not be punitive, but at the same time it’s clear that Europe knows how to defend its interests, and that Britain will have a less good position outside the EU than in the EU.”
The comments were echoed by the Belgian prime minister, Charles Michel, who said there was no such thing as a “free Brexit”.
Asked about the British prime minister’s claim that she will be strengthened by an election victory, Hollande, who is now in his last week as president, said: “That is an election argument that I can understand, but this is not an argument against the European Union. Why? Because the bases, the principles, the objectives are already fixed. These will be the lines that will be chosen by the negotiators and there will be no others.”
Luxembourg’s prime minister, Xavier Bettel, also ruled out the idea of Theresa May gaining any advantage from an election win. “It’s an internal problem she wants to resolve in the Conservative party, to have not a hard Brexit or a soft Brexit, but Theresa’s Brexit,” he said. “We are very united. You seem surprised, but it’s a fact.”
The EU’s chief Brexit negotiator, Michel Barnier, said it was in Britain’s interests for the EU to be unified, as it would boost the chances of a deal.
“This extraordinary meeting shows the unity of the 27 on a clear line, but this unity is not directed against Britain, I think that it is also in its interest,” he said.
The Dutch prime minister, Mark Rutte, said that only once there was sufficient trust between the EU nations and Britain on the core issues could both sides proceed to discuss future relations.
Rutte, who stressed the importance of Britain to the Dutch economy, told reporters that “as you get to a certain level, as far as possible, and say now we are confident about this, then we have to swiftly start talking about the future relationships trade and also politics”.
The German chancellor, Angela Merkel, had earlier in the week accused some in the UK of being deluded about the consequences of leaving the EU, but offered a softer tone in Brussels.
“We want to have good relations with Great Britain in the future, but we want to represent our interests as the 27. So far this has been a real success,” she said.
“We will hold the negotiations on separation first and then at a certain point when the substantive points in the separation negotations we will come to a point when we can talk about the future.
“The separation negotiations on the rights of citizens of our states in the UK and of UK citizens in the EU. Also financial questions are part of the separation questions.”
There is some pessimism in Brussels about the prospect of a deal being struck over the next two years, though some EU officials have taken heart from the fact that May has not recently repeated her claim that “no deal is better than a bad deal” despite being pushed to do so by politicians in favour of a hard Brexit.
“We are convinced that no deal is in no one’s interest. We appreciate the fact that the tone of the debate in the UK on this issue has changed,” said a senior EU official on Friday.
Asked, however, to respond to May’s claims on the general election campaign trail that member states were preparing to “line up to oppose us”, one senior EU diplomat said: “She’s right. She should not underestimate the commitment to unity.”
Key negotiating points for the EU27
1. There will be no “cherry picking” of the four core single market freedoms, which are “indivisible”. This rules out government’s hope of keeping “elements of a single market” without free movement of people.
2. The 27 countries will negotiate with Britain as a unified block, relying on the principle that “nothing is agreed until everything is agreed”. No individual countries will be allowed backchannel chats on future trade deals while the transition is still being talked through.
3. There should be a phased approach to Britain’s withdrawal. The first phase will set out to “avoid disruption” from an “abrupt change”, and the main priority will be to give certainty to EU citizens about their legal status. Once that is decided, the European council will give the go ahead for next phase of withdrawal, which would involve working out a framework for the future relationship.
4. Citizens’ rights will be the “first priority of the negotiations”. The rights of EU and UK citizens will be protected when the UK formally leaves, which at this point looks as if it will be around March 2019. This means that any EU national who has been living in the UK for five years by that point will be protected.
5. “Flexible and imaginative solutions” will be sought to the thorny issue of the border between Northern Ireland and the Republic, which has been fluid since the Good Friday agreement. Negotiations will aim to avoid establishing a hard border, which it is feared could trigger a return to violence.
6. No final figure for the divorce has been given, but the bloc will pursue one “single financial settlement”, meaning the UK will continue budget payments until 2020.
EU Leaders Agree ‘Firm’ Brexit Negotiating Stance
European Union leaders have unanimously agreed to adopt a “firm” negotiating approach for Brexit talks with the UK.
On Saturday the heads of the remaining 27 states approved within a minute the draft guidlines issued by European Council President Donald Tusk on 31 March.
Tusk tweeted that the mandate agreed by the 27 country leaders was also “fair”.
Talks with the UK will begin after the General Election on June 8.
The negotiations must be completed by the deadline March 29 2019.
In his pre-summit comments Tusk said resolving uncertainty around citizens’ rights was the EU and UK’s “number one priority”.
In regard to the financial settlement, the final guidelines provide more detail on the “obligations and liabilities” the UK will be asked to cover.
It lists issues resulting from the MFF (the EU’s long term spending plan) as well as “those related to the European Investment Bank (EIB), the European Development Fund (EDF) and the European Central Bank (ECB)”.
The paragraph listing the other areas of potential future co-operation, aside from trade, has added “foreign policy” to security, defence and the fight against terrorism and international crime.
A paragraph has also been inserted stressing that any future UK-EU relationship should “safeguard financial stability in the Union and respect its regulatory and supervisory regime and standards and their application”.
In recent weeks debate has flared around whether a free trade deal would include the financial services industry and, if it did, whether City of London institutions would still be bound by Brussels oversight.
The paragraph on the Irish border has made clear the EU’s support of the 1998 Good Friday peace agreement.
Some structures agreed in what is an international accord between the UK and Republic of Ireland were based on EU law and the UK will now face pressure to ensure post-Brexit laws in Northern Ireland do not undermine the terms of the peace deal.
One of the more controversial elements of Tusk’s draft guidelines in March was a suggested veto for Spain on any future UK/EU agreements that involved Gibraltar.
There has been no change to the wording of that paragraph.