It's the first time EU members have met since the United Kingdom held its referendum on June 23rd.
As he arrived in Brussels to explain how his Brexit gamble failed, British Prime Minister David Cameron said he hope the talks would be constructive.
"I'll be explaining that Britain will be leaving the European Union but I want that process to be as constructive as possible and I hope the outcome can be as constructive as possible because of course while we're leaving the European Union, we mustn't be turning our backs on Europe."
German Chancellor Angela Merkel had earlier warned that the EU would not tolerate Britain leaving yet trying to retain the benefits of EU membership.
"There must be a very clear difference if the country wants to remain part of the EU family or not. Whoever wants to exit from this family cannot expect to be exempt from all the obligations and yet still benefit from all the privileges."
David Cameron, who's stepping down as Prime Minister as a result of the outcome, says he'll leave it to his successor to formally begin the exit process.
But European Commission President Jean Claude Juncker says that delay comes at a price.
"I have placed a Presidential ban - I don't like to do that - on Commissioners engaging in discussions with the British government, regardless of whether it was leave or remain. I said to the Director General the same - that they can have no preliminary discussions with representatives. No notification, no negotiations."
Meanwhile, there were fiery exchanges in the European Parliament, with UKIP leader Nigel Farage, a key figure in the campaign for Britain to leave the EU, the focus.
"You know, when I came here 17 years ago and I said that I wanted to lead a campaign to get Britain to leave the European Union, you all laughed at me. Well, I have to say you're not laughing now. And the reason you're so upset, and the reason you're so angry, has been perfectly clear from all the angry exchanges this morning - you as a political project are in denial."
That prompted an angry reaction from the EU President..
"You lied. You didn't tell the truth. You fabricated reality. That's the last time you are applauding here. And to some extent I am really surprised that you are here. You were fighting for the exit, the British people voted in favour of the exit. Why are you here?!"
Manfred Weber, from the European Peoples' Party, didn't hold back.
"The worst liars can be found among UKIP. On Friday Nigel Farage said publicly that the promised 350 million pounds a week would finally not go to the National Health Service. It had all been a lie. I ask Mr Farage, if you had an ounce of decency in you, you would apologise today to the British. Shame on you."
Guy Verhofstadt, from the Alliance of Liberals and Democrats for Europe, attacked the tone of the British campaign.
"What makes it so hard for me, and I think also for the other groups here, and for everybody here in this House, is the way it suceeded. The absolute negative campaigning. The posters of Mr Farage showing refugees like in Nazi propaganda ....."
And then this impassioned plea from Scottish MP Alyn Smith.
"Now colleagues there is a lot of things to be negotiated. We will need cool heads and warm hearts. But please remember this: Scotland did not let you down. Please, I beg you, do not let Scotland down now!"
