Around a dozen policemen, some wearing stab vests, were positioned outside the embassy in the exclusive Knightsbridge district of London near the Harrods department store.

Assange, an Australian ex-computer hacker, has been holed up in the embassy since June 19, when he claimed political asylum in a bid to avoid extradition to Sweden where he faces questioning over alleged sex crimes.

Around a dozen protesters also stood outside the embassy early Thursday after Britain threatened to storm the building and arrest Assange, whose website published hundreds of thousands of secret US government documents.

A few activists camped out overnight outside the embassy.

The protest's Facebook page claimed another 600 more demonstrators were expected later at the embassy and have threatened to "occupy" it.

"This situation is contradictory in a country which heralds free speech," an 18-year-old protester who gave her name as Ella told AFP.

"What he (Assange) did is beautiful and important. We need to show solidarity."

Ecuador has hit out at Britain for threatening to breach its embassy's diplomatic immunity and enter the building to arrest the 41-year-old Australian, while WikiLeaks said such action would be "a hostile and extreme act".

Assange, who fled to the embassy after reaching a dead end in his marathon British legal battle to avoid extradition, says he fears he could eventually be passed on to the US if he was sent to Sweden.

The US has mulled action against Assange after WikiLeaks published hundreds of thousands of confidential US files on the wars in Iraq and Afghanistan as well as a vast cache of embarrassing US diplomatic cables.

Assange's supporters say he could even face the death penalty in the US, pointing to authorities' harsh treatment of US army private Bradley Manning, who is on trial for allegedly leaking secret military documents to WikiLeaks.