"There will inevitably be a price and a cost for Britain, it's the choice they made," Hollande said as he arrived at a Brussels summit.
"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."
Hollande, who is entering his last days as French president, dismissed suggestions that British Prime Minister Theresa May could strengthen her negotiating hand by winning a big mandate in elections that she has called for June 8.
"I can understand the electoral argument but it will not influence the EU. The EU's principles and the objectives are already fixed, these will be the lines chosen by negotiators."
Luxembourg Prime Minister Xavier Bettel also ruled out an advantage for May from a big 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 meanwhile said it was also in Britain's interests for the EU to be unified, as it would boost the chances of a Brexit 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.
Meanwhile, European Union leaders called for a united front, a call that comes hot on the heels of a war of words between May and German Chancellor Angela Merkel, who said Britain should not have "illusions" about the talks.
On arrival at the summit, Merkel said the EU wants "good relations" with Britain but added that "we also want to defend, at 27, our common interests -- so far we have done extremely well."
The EU's chief Brexit negotiator, Michel Barnier, said the bloc had a "clear line" on the talks. "But this unity is not directed against Britain, I think that it is also in its interest," he said.
The EU 27 have considerably toughened the guidelines since Tusk first unveiled them a month ago, with Brussels also drawing up a detailed list of citizens' rights.
Tusk said Europeans needed "solid guarantees for citizens and their families, who will be affected by Brexit on both sides -- this must be number-one priority for EU and the UK."
This referred to the fate of three million EU citizens living in Britain and one million Britons on the continent, with officials hoping for a resolution on their status after Brexit by the end of the year.
In a further move that will rile London, the EU is also set to back automatic membership for Northern Ireland if it reunifies with Ireland, and call for Spain to have a say over any deal that affects Gibraltar.
The leaders will also discuss for the first time the spoils of Brexit -- the relocation of EU medical and banking agencies that are currently based in London.
Virtually all of the 27 have put their hand up to win one of the agencies.
While the EU says citizens' rights is a priority, the most touchy issue of all is likely to be Britain's exit bill.
This is estimated at around 60 billion euros which mainly covers financial commitments made by the bloc while Britain was a member.
The bill is politically toxic for Britain but also risks causing divisions among EU states as they debate how to plug any holes in the EU's budget.