WikiLeaks founder Julian Assange appeared in a British court on Monday to fight attempts to extradite him to Sweden over sex crime allegations.

Lawyers for the 39-year-old Australian are expected to argue that the extradition request is unacceptable because he has not been charged with any crime.

Wearing a grey coat and dark blue suit and tie, Assange smiled as he entered the complex at Belmarsh Magistrates' court in Southeast London, Britain's highest security court, for the start of the two-day hearing.

A decision is not expected on Tuesday, with the judge expected to defer until later on February.

If the ruling goes against Assange he will be able to appeal all the way to England's Supreme Court.

Swedish prosecutors want to question Assange over allegations of sexual assault and rape made by two women.

Assange, who won worldwide notoriety for his website's release of thousands of secret US diplomatic cables, insists the attempts to extradite him are politically motivated.

His lawyers were to argue that if Assange were extradited to Sweden, he would risk extradition or could even be passed on to the United States where they say he could face the death penalty.

Assange, who was arrested in London in December 7, now faces a widening criminal probe in the United States and has made powerful enemies in Washington.

The former computer hacker's legal team were to take the rare step of publishing their defence argument in full online later on Monday.

One of his lawyers, Mark Stephens, told The Daily Telegraph newspaper: "You will see some fundamental challenges to the European arrest warrant scheme."

Lawyers are expected to argue that the arrest warrant is invalid because Assange is only wanted for questioning and has not been charged and that the Swedish prosecutor lacked the authority to issue it.

Assange was released on bail a week after his arrest and has since been staying at a supporter's country mansion, under strict conditions including that he obey a curfew, wear an electronic ankle tag and report to the police daily.

Leaked details have cast new light on the rape and molestation accusations he faces after Swedish police reports filled with graphic details of the allegations reached the Internet last week.

The police documents, viewed by AFP, contain a statement from the alleged rape victim alleging that Assange forced himself on her, without wearing a condom, while she was asleep.

The woman, identified only as Miss W, said she had had consensual sex with Assange earlier in the evening and had then fallen asleep with him, only "to wake up because he has forced himself inside of her," the report said.

"'She asked immediately: are you wearing anything?' and he answered 'you'," it added.

"She told him 'You better not have HIV,' and he answered 'Of course not'."

After that, Miss W allowed the intercourse to continue.

The documents also include a forensic report on the condom used during a sexual encounter with Assange's other alleged victim, Miss A, who accused him of having deliberately broken the prophylactic.

The report says the condom had not been cut with scissors or a knife.

Celebrity backers including socialite Jemima Khan were planning to lead rallies in London for Assange during his extradition hearing.

Khan, the wealthy former wife of Pakistan cricketer Imran Khan, was in court for the start of the hearing.

After releasing hundreds of thousands of confidential US documents about the wars in Iraq and Afghanistan last year, WikiLeaks has in recent months been slowly publishing more than 250,000 leaked US diplomatic cables.