Mr Tusk delivered a boost to campaigners calling for a referendum on the terms of the UK's Brexit deal, telling MEPs gathered in Strasbourg: "Wasn't it David Davis himself who said 'If a democracy cannot change its mind, it ceases to be a democracy'?"
The European Council President said: "If the UK Government sticks to its decision to leave, Brexit will become a reality - with all its negative consequences - in March next year unless there is a change of heart among our British friends.
Mr Tusk said the EU had not had a "change of heart" over Brexit, but added: "Our hearts are still open for you."
Mr Verhofstadt told MEPs: "I don’t know what he put in the coffee or the tea of Nigel Farage because he comes out of this meeting and backs a second referendum."
Edinburgh MP Ian Murray, who has called for the UK to stay in the European single market after Brexit, said: “Until we've actually left, Brexit is a reversible process, that much is clear. If people decide that Brexit isn't the right path for the country, they have the right to change their minds.
“The point has been made by legal experts, EU leaders and even the architect of Article 50, Lord Kerr. The Government should now be upfront with the public and publish its own legal advice on the matter."
Repeating the comment by Brexit Secretary David Davis, Mr Murray added: "For once, he's right.”
In Strasbourg, Mr Turks called for continued unity among the remaining 27 members of the EU, saying: "The hardest work is still ahead of us and time is limited. We must maintain the unity of the EU27 in every scenario, and personally I have no doubt that we will."
Mr Tusk's comments came as a leaked European Commission paper suggested the EU is toughening its stance on the transition period after the official date of Brexit in March 2019.
A document obtained by The Guardian suggested the EU will insist on free movement of people throughout the period, and permanent rights to settle for any EU nationals moving to the UK before the end of 2020.
EU leaders have also set up a clash between the UK Government and fishermen, with Brussels taking a tougher stance on the UK staying within the Common Fisheries Policy during the transition phase.
Chief negotiator Michel Barnier is set to insist that the UK will have to seek "authorisation" from Brussels to continue enjoying the benefits of the bloc's existing trade agreements with non-EU countries, the paper suggests. Reports on Tuesday suggested that UK negotiators have already asked for that permission as part of a transition phase.
Mr Tusk said: "As regards our future relations, what we need today is more clarity on the UK's vision. Once we have that, the leaders will meet and decide on the way the EU sees its future relationship with the UK as a third country."
And European Commission president Jean-Claude Juncker told the parliament in Strasbourg that he hoped Mr Tusk's message "will be heard clearly in London".