EU Commission President Jean-Claude Juncker has accused Brexit campaigners Boris Johnson and Nigel Farage of quitting when things got difficult.
"The Brexit heroes of yesterday are now the sad Brexit heroes of today," he told the European Parliament.
There was anger among MEPs over the UK's 23 June vote to leave the EU.
Mr Juncker spoke of Leave camp "retro nationalists". "Patriots don't resign when things get difficult, they stay," he told MEPs in Strasbourg.
He also said he did not understand why those in the Brexit camp in the UK would want to wait before beginning the formal withdrawal process.
"Instead of developing the plan, they are leaving the boat," he said.
Boris Johnson, former London mayor and a leading Brexit campaigner, caused a sensation last Thursday when he pulled out of the Conservative leadership race.
He had been considered a favourite to replace David Cameron as party leader and prime minister.
UK Independence Party (UKIP) leader Nigel Farage resigned on Monday, saying he wanted his "life back", as he had fulfilled his political ambition of putting the UK on a path to exit from the EU, nicknamed "Brexit".
Mr Farage has clashed repeatedly with Mr Juncker in the European Parliament.
Brexiteers accuse the Commission of dictating policy to the UK and expanding its powers to the detriment of Europe's citizens. The Commission drafts EU laws and polices the bloc's regulations.
Once the Leave camp's victory was clear, David Cameron resigned, having led the Remain camp's attempt to keep the UK in the 28-nation bloc.
Dutch Prime Minister Mark Rutte told MEPs that the Brexit vote was "extremely unfortunate", especially for the UK. "That country now has collapsed - politically, economically, monetarily and constitutionally, and you will have years ahead of you to get out of this mess."
The referendum result caused political and economic turmoil in the UK. The pound slumped to a 31-year low against the US dollar and there are fears that the UK is heading for a recession again.
The result also hit stocks, especially banks and housebuilders.
UKIP deputy leader Paul Nuttall told MEPs that "the UK and Europe are joined by geography, culture, history and trade and that will not change".
He said "it's imperative that we are good, healthy, trading neighbours". "We need negotiations conducted in a grown-up manner so we can get the best deal for everyone."
EU Council condemned
The head of the liberal group in the parliament, former Belgian Prime Minister Guy Verhofstadt, lambasted the European Council, the forum where EU governments decide policy.
He said the Council's reaction to Brexit was "we shouldn't change anything, just implement existing European policies". "I find this shocking and irresponsible," he said angrily.
There had been warning signs for the EU from previous referendums in Denmark and the Netherlands, he said.
"What are you waiting for? When will the Council recognise that this type of EU - you cannot defend it any more. Europe needs to be reformed... European citizens are not against Europe, they're against this Europe."