Julian Assange remained holed up in Ecuador's London embassy on Friday with police guarding its exits, as Britain warned that the diplomatic row over the WikiLeaks founder could go on for years.

Ecuador granted asylum on Thursday to Assange - whose website enraged the United States by publishing a vast cache of confidential government files - but Britain has vowed not to grant him safe passage out of the country.

British Foreign Secretary William Hague said his government was obliged under its own law to extradite the Australian national to Sweden, where he is wanted for questioning over alleged sex crimes.

"No-one, least of all the government of Ecuador, should be in any doubt that we are determined to carry out our legal obligation to see Mr Assange extradited to Sweden," Hague told a press conference on Thursday.

He admitted that the stalemate could continue for months or even years.

Around ten British police were stationed outside the embassy early on Friday, ready to arrest 41-year-old Assange if he leaves the building.

WikiLeaks condemned the continued police presence as 'intimidation tactics'. A handful of Assange's supporters camped overnight outside the embassy in London's plush Knightsbridge district, sleeping on pieces of cardboard.

Assange has not left the embassy since June 19, when he walked in and claimed asylum.

WikiLeaks said on Twitter that he would give a statement in front of the embassy on Sunday, though it did not specify whether this would involve leaving the building and, if so, how he would do this without being arrested.

Under normal diplomatic procedures, embassies are considered the territory of the countries they represent and cannot be entered without permission.

But Britain angered Ecuador on Thursday by suggesting it could invoke a domestic law allowing it to breach the usual rules and go in to arrest Assange.

WikiLeaks spokesman Kristinn Hrafnsson on Thursday warned Britain against any attempt to enter the building, saying this would "risk upsetting diplomatic relations all over the world".

"Hopefully, we will see the decision resolved in a civilised manner," he told AFP.

Entering the embassy without Ecuador's permission would challenge a fundamental principle of diplomacy, and the threat has left Britain in unchartered legal waters.

But Britain's representative in Ecuador, Philip Barton, said London was "committed to finding a mutually acceptable solution to this problem."

A spokeswoman for Britain's Foreign Office declined to say whether meetings would be held with Ecuadorean officials on Friday to attempt to end the impasse.

The Organization of American States, which has held an emergency session, said it would decide Friday whether to call a meeting of its foreign ministers. Britain has observer status in the OAS.

Ecuador has called a meeting of foreign ministers from the South American regional bloc UNASUR on Sunday.

"Nobody is going to scare us," Ecuador's President Rafael Correa said on his Twitter account, minutes before the decision was announced.

Ecuadorean Foreign Minister Ricardo Patino said his government reached its decision after Britain, Sweden and the United States refused to provide guarantees that Assange would not be passed on from Sweden to the US.

In 2010, WikiLeaks obtained and published online an enormous cache of US military documents on the war in Iraq and Afghanistan and diplomatic cables that deeply embarrassed the US.

Supporters fear Assange could face the death penalty if sent to Washington, pointing to its harsh treatment of Bradley Manning, the soldier on trial for allegedly leaking hundreds of thousands of military files to WikiLeaks.

Washington has denied that it is lobbying Britain or Sweden to take Assange into custody.

"With regard to the charge that the US was intent on persecuting him, I reject that completely," State Department spokeswoman Victoria Nuland said.

"It is an issue among the countries involved and we are not planning to interject ourselves."

Assange, meanwhile, thanked Ecuador on Thursday for its "courageous" decision.

"While today is a historic victory, our struggles have just begun. The unprecedented US investigation against WikiLeaks must be stopped," he said.