WikiLeaks founder Julian Assange appealed to Britain's Supreme Court on Wednesday not to extradite him to Sweden over accusations of sex crimes, a transfer that could push his anti-secrecy website further towards oblivion.

Australian-born Assange, 40, became a worldwide celebrity and infuriated the U.S. government in 2010 when WikiLeaks released secret video footage and thousands of U.S. diplomatic cables about the wars in Iraq and Afghanistan.

Since then, WikiLeaks has faded from the news headlines, starved of cash by major credit card companies that are refusing to process online donations to it, and bogged down by Assange's protracted legal troubles.

He was detained in Britain in December 2010 on a European arrest warrant issued by a Swedish prosecutor after two female former WikiLeaks volunteers accused him of sexual assault. He has not been charged but is wanted for questioning.

His British lawyers argue that the warrant is invalid because it was issued by a prosecutor, not an impartial judge or court. That is the point being argued before the Supreme Court, which will not look into the substance of the allegations against Assange.

Dressed in a dark grey suit and purple tie, Assange sat quietly behind his legal team, taking notes and reading documents. He did not speak to waiting reporters when the hearing ended for the day.

The case will last two days, and the court is expected to announce its decision some weeks later.

A small group of demonstrators stood outside the Supreme Court during the hearing, braving freezing weather to show support for Assange, with banners bearing slogans like The system punishes whistleblowers to protect its own and Free Assange.

SPEAKS TRUTH TO POWER

He speaks truth to power. He should be considered a national hero, said Scott Albrecht, a former U.S. soldier turned peace activist.

If the Supreme Court rejects Assange's appeal, he could take his case to the European Court of Human Rights in Strasbourg. If that court agrees to consider his case, it can direct Britain not to hand him over to Sweden until its proceedings are over.

Should the Strasbourg court decline to take on his case, Britain could extradite him to Sweden as soon as logistics allow.

Assange denies any wrongdoing and has said that the sex accusations, which surfaced at the height of the international furore over WikiLeaks, were an attempt to silence him.

He is convinced that U.S. authorities are looking for a way to pursue him in retaliation for WikiLeaks' revelations.

Views in Washington are divided, some officials calling for tough action against Assange to deter would-be leakers and others saying a prosecution would be legally problematic and would give him a boost just when he appears headed for irrelevance.

Bradley Manning, a U.S. army intelligence analyst suspected of passing thousands of classified documents to WikiLeaks, is facing a court martial on 22 charges including aiding the enemy and wrongfully causing intelligence to be published online.

Assange has been holed up under house arrest for over a year, mostly at the English country mansion of a wealthy supporter.

He has made some unexpected career moves, including a guest appearance in an episode of the satirical U.S. animated series The Simpsons due to be aired on February 19.

In another TV first, the Kremlin-funded station Russia Today announced last week that it had hired Assange to host a political talk show called The World Tomorrow.

(Editing by Tim Pearce)