EU states have urged Britain to hold speedy talks on leaving the bloc after it voted to end its membership in a historic referendum.
German Foreign Minister Frank-Walter Steinmeier said negotiations should begin as "soon as possible".
He made the comments after an urgent meeting of the six EU founder members to discuss the decision.
British PM David Cameron has said he will step down by October to allow his successor to conduct talks.
The six countries attending the summit in Berlin - Germany, France, Italy, Belgium, Luxembourg and the Netherlands - first joined forces in the 1950s and still form the core of the EU.
"We say here together, this process should get under way as soon as possible so that we are not left in limbo but rather can concentrate on the future of Europe," Mr Steinmeier said.
His Dutch counterpart Bert Koenders said the continent could not accept a political vacuum, saying "this will not be business as usual".
Meanwhile on negotiations, German Chancellor Angela Merkel said: "It shouldn't take forever, that's right, but I would not fight for a short time frame."
She added that she was seeking a "objective, good" climate in talks on Britain's exit from the EU, and that there was no need to make deterrence a priority.
Mrs Merkel also said there was "no need to be particularly nasty in any way in the negotiations; they must be conducted properly".
In other developments:
- Scotland's First Minister Nicola Sturgeon says she will seek "immediate discussions" with Brussels to "protect Scotland's place in the EU"
- A petition calling for a second referendum on UK's membership of the EU has gained more than one million signatures
- There are warnings British financial institutions could lose their prized access to the EU if the UK leaves the common market
- Britain's European Commissioner, Lord Hill, who oversees financial services, is resigning
The first summit of EU leaders with no British representation will be held on Wednesday, a day after Mr Cameron holds talks with members.
Global stock markets and the pound fell heavily on the news of the so-called "Brexit", while credit rating agency Moody's cut the UK's outlook to "negative".
The UK must now invoke Article 50 of the EU Lisbon Treaty, which sets out a two-year timetable for negotiations on withdrawal.
European Commission head Jean-Claude Juncker said the EU-UK split was "not an amicable divorce", but nor had they had a "deep love affair". He has also said exit negotiations should begin immediately.
"Britons decided yesterday [Thursday] that they want to leave the European Union, so it doesn't make any sense to wait until October to try to negotiate the terms of their departure," Mr Juncker said in an interview with Germany's ARD television network.
Germany wakes up to Brexit, by Jenny Hill, BBC News, Berlin
"OUTsch!," "Well done little Britain," "Completely detached."
Germany's press reflects the widespread shock at the British decision to leave the EU. At the highest level of government no-one really expected this. And they are scrambling to defend German and European interests.
The German position is focused on two major, and delicate, decisions.
Firstly, whether to use Brexit as an opportunity to deepen European integration, or to view it as a "wake-up call" and create more flexibility within the union.
And, secondly, how to deal with Britain - an important trading partner - as a 'third country.'
Senior economists and business leaders warn against barriers to free trade.
But politicians are talking tough. Concessions, they say, might encourage other member states to leave. For this reason one senior MP told me "there must be consequences for Britain".
How European powers reacted
- German Chancellor Angela Merkel expressed "great regret" at the British decision, saying: "This is a blow to Europe and to the European unification process"
- French President Francois Hollande said the vote "seriously puts Europe to the test" but called for the EU to push on with reform and investment
- Italian PM Matteo Renzi, who will meet Mr Hollande on Saturday, said: "Europe is our house," adding that "the house needs to be renovated, perhaps freshened up"
- Russian President Vladimir Putin said the decision showed the UK's unhappiness with migration and security
- Greek PM Alexis Tsipras said the vote was "either a wake-up call or the beginning of a dangerous path", adding: "We urgently need a new vision and beginning for a united Europe"
Other leaders, including Polish President Andrzej Duda, said it was crucial that work be done to prevent other countries leaving the EU.
Turkey's President Recep Tayyip Erdogan said it was inevitable more countries would leave the EU soon if it did not change its path.
Leaders of Eurosceptic parties in France, the Netherlands and Italy quickly demanded referendums in their own countries.
What comes next?
What is Article 50 of the EU Treaty?
- In force since 2009 but never tested
- Allows governments to notify intent to leave. Talks then begin on a range of issues between the leaving nation and other EU members
- If no deal is reached, membership will automatically cease two years after notification
- The article is only a basic template for leaving, settling the date and some other matters. It does not automatically include issues such as movement of people or trade. The latter could take years to conclude