A stunned European Union has urged Britain to leave as "soon as possible" amid fears the devastating blow to European unity could spark a chain reaction of further referendums.

Key points: European leaders are calling for urgent reform after Britain's exit

European leaders are calling for urgent reform after Britain's exit French President Francois Hollande called the outcome 'a grave test for Europe'

French President Francois Hollande called the outcome 'a grave test for Europe' Far-right leaders in France and the Netherlands want similar referendums

German Chancellor Angela Merkel and French President Francois Hollande led calls for the European Union to reform in order to survive a traumatic divorce with Britain, after 52 per cent of voters there opted to leave the EU.

In a sign that the bloc wants to move on swiftly, EU chiefs told Britain in a strongly-worded joint statement to "give effect to this decision of the British people as soon as possible, however painful that process may be".

The uncompromising stance came after Prime Minister David Cameron said he would resign and leave the negotiations on Britain's departure from the 28-nation club to a successor who will be named by October.

Worried European leaders will hold a series of crisis talks in coming days, with Ms Merkel saying she would host the leaders of France and Italy along with EU President Donald Tusk in Berlin on Monday to try to chart a reform plan.

Before that, the foreign ministers of Germany, France, Italy, Belgium, Luxembourg and the Netherlands — the founding members of the EU — will meet in Berlin on Saturday.

"We take note of the British people's decision with regret," Ms Merkel told reporters in Berlin.

"There is no doubt that this is a blow to Europe and to the European unification process."

With global markets in turmoil, Ms Merkel said it was important to "not draw quick and simple conclusions from the referendum in Great Britain, which would only further divide Europe."

Mr Hollande said the Brexit vote was a "grave test for Europe," adding that the bloc "must show solidity and strength in its response to the economic and financial risks".

He said the British should leave "with the shortest possible delay".

Greek Prime Minister Alexis Tsipras, whose debt-hit country went through a referendum on its way to a bitterly-fought debt bailout, said the result could be a "wake-up call" for the EU to "change its policies".

EU President Donald Tusk, who had earlier warned that a Leave vote could "end Western political civilisation" — put on a brave face, saying that "what does not kill you makes you stronger".

Boris Johnson booed in London

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Boris Johnson, the former London mayor who became a key figure in the Leave campaign and is tipped to replace Mr Cameron as PM, was booed as he left his London home in the wake of the vote.

Protesters shouted "Shame on you Boris" and "You're a parasite".

A few hours later, Mr Johnson fronted a press conference as the gravity of what he had done sunk in.

"This does not mean that the United Kingdom will be in any way less united; nor, indeed, does it mean that it will be any less European," he insisted.

"We cannot turn our backs on Europe, we're part of Europe. Our children and grandchildren will continue to have a wonderful future as Europeans," he added.

He said the European Union was "a noble idea for its time" but was "no longer right for this country".

'Chain reaction' fear

The biggest fear in Europe is of contagion, with immediate calls by far-right leaders in France and the Netherlands for their countries to hold their own votes on EU membership.

French far-right leader Marine Le Pen said the British result was a "victory for freedom", while Dutch anti-Islam MP Geert Wilders said "the Dutch people deserve a referendum as well".

Donald Trump, the presumptive Republican US presidential nominee, also weighed in with his support while on a visit to Scotland, saying the vote for Brexit was a "fantastic thing," evoking a "real parallel" with his own campaign.

European Parliament President Martin Schulz said he was speaking to Ms Merkel to avoid a "chain reaction" of eurosceptic success across Europe, adding that it would "absolutely not happen".

Several European leaders expressed "sadness", including Spanish Prime Minister Mariano Rajoy, whose country goes to the polls on Sunday in yet another test for Europe's traditional politics.

The 27 remaining EU leaders will meet separately without Mr Cameron in Brussels next week to discuss Britain's exit.

The European Parliament will meanwhile pass a motion at an emergency meeting on Tuesday urging Mr Cameron to trigger the exit process by invoking what is known as Article 50.

AFP/ABC