LONDON (AFP) - More than a million Britons pleaded for a second referendum Saturday (June 25) as Britain's seismic vote to abandon the EU split the nation after pounding world markets, toppling the prime minister and raising the threat of a breakup of the island nation.

In a sign of the fissures exposed by the June 23 vote, 1.2 million people signed a petition on the official government website by late morning calling for a repeat vote - more than 12 times the 100,000 signatures required for a proposal to be discussed in the lower house of parliament.

Unprecedented traffic forced the site to be taken out of action at one point, a parliamentary spokesman said.

A parliamentary committee, which can put forward petitions for debate by lawmakers, will consider the proposal Tuesday (June 28).

"I am worried, really sick for my children's prospects," said Lindsey Brett, a 57-year-old secretarial worker.

"I was expecting a 'Remain' vote. I did not think we would come out," she said in central London.

Britons, many worried about immigration and financial insecurity, cast aside Prime Minister David Cameron's warnings of isolation and economic disaster and voted 52 per cent-48 per cent in favour of "Brexit" in Thursday's referendum.

Their decision pounded sterling and global stock markets. Moody's cut Britain's credit rating outlook to "negative", warning of the economic threat to the country.

Cameron announced Friday he would resign by October and let his successor lead the exit negotiations under Article 50 of the Lisbon Treaty, which sets out a two-year time-frame to leave.

European powers called for Britain to be shown the door quickly as they grappled with the impending loss of one of the world's top economies, the first defection in the bloc's 60-year history.

Brexit negotiations must take place "quickly and swiftly", EU Commissioner Pierre Moscovici told Britain's Radio 4 on Saturday.

"I do not understand why the British government needs until October to decide whether to send the divorce letter to Brussels," Juncker told German broadcaster ARD on Friday evening.

"I would like it immediately," he added. "It is not an amicable divorce but it was also not an intimate love affair."

Foreign ministers of the six original EU members - Germany, France, Italy, the Netherlands, Belgium and Luxembourg - gathered in Berlin for the first in a series of emergency meetings over the next week triggered by Britain's decision.

"We join together in saying that this process must begin as soon as possible so we don't end up in an extended limbo period but rather can focus on the future of Europe and the work toward it," said German Foreign Minister Frank-Walter Steinmeier as he entered the meeting at a lakeside villa.

The Franco-German axis at the heart of the bloc, which was born out of a determination to forge lasting peace after two world wars, will propose "concrete solutions" to make the EU more effective, French Foreign Minister Jean-Marc Ayrault told AFP.

He, too, called on London to move more swiftly. EU leaders will open a two-day Brussels summit on the crisis on Tuesday. In an early sign of the Brexit fallout in Brussels, Britain's European commissioner for financial services, Jonathan Hill, said he would stand down.

"I don't believe it is right that I should carry on as the British commissioner as though nothing had happened," he said in a statement.

Britain faced a historic break-up threat, too, as Scotland stood aghast at the prospect of being dragged out of the 28-nation European Union when more than 60 per cent of its people voted to stay in.

"A second independence referendum is clearly an option that requires to be on the table," First Minister Nicola Sturgeon declared after an emergency meeting of Scotland's parliament, which agreed to start to draw up legislation that could enable such a vote once a decision is taken.

Scotland is seeking "immediate discussions" with its EU partners to try to protect its position in the bloc, she said. Scots backed staying in Britain in their last referendum in 2014.

The EU referendum, the culmination of an often poisonous campaign, revealed divides across British society, including between what The Independent newspaper called "those doing well from globalisation and those 'left behind' and not seeing the benefits in jobs or wages".

Young people, graduates, and big cities tended to favour "Remain". Elder, less educated people and rural populations were more likely to back "Brexit".

Britain's rejection of the EU is being seen as a victory for the anti-establishment rhetoric of the Brexit campaign, a feature of growing populism across Europe.

"Take a bow, Britain!" eurosceptic newspaper the Daily Mail wrote across its front page on Saturday.

"It was the day the quiet people of Britain rose up against an arrogant, out-of-touch political class and a contemptuous Brussels elite," it added.

The British vote will stoke fears of a domino-effect of exit votes in eurosceptic member states that could imperil the integrity of the bloc.

Dutch far-right MP Geert Wilders and French National Front leader Marine Le Pen immediately called for referendums on EU membership in their own countries.