European leaders said on Wednesday that they would remain united and strive to protect the bloc's interests after Britain formally triggered the process of leaving the union.
Soon after the UK Prime Minister Theresa May formally notified Brussels about London's decision, European leaders expressed sadness and commitment to hold the 27-nation bloc together.
European Council President Donald Tusk told reporters it was an unhappy day for both London and Brussels.
"There is no reason to pretend today is a happy day, neither in Brussels nor in London. After all most Europeans, including almost half the British voters, wished that we would stay together, not drift apart," he said.
TRT World's Europe correspondent Simon McGregor-Wood reports on this from London.
Tusk's briefing was followed by a joint statement by EU leaders.
"The Union will act as one and preserve its interests. Our first priority will be to minimise the uncertainty caused by the decision of the United Kingdom for our citizens, businesses and member states, " the statement said.
Denmark's Prime Minister Lars Loekke Rasmussen said in a statement that Britain's "goodbye" to the EU is "incredibly sad," adding that he expects "many bumps on the road," and that he hopes "the divorce" will take place in "an orderly fashion."
German Chancellor Angela Merkel said there could be no negotiations on Britain's future ties with EU until the Brexit terms were finalised.
"The negotiations must first clarify how we will disentangle our interlinked relationship ... and only when this question is dealt with, can we, hopefully soon after, begin talking about our future relationship," Merkel said.
During a visit to Indonesia, French President Francois Hollande said while Brexit is "sentimentally painful" for the Europeans, it would be "economically painful" for the British.
But as a matter of principle "it is not our intention to punish [the British]."