WikiLeaks founder Julian Assange has again appealed for Australian diplomatic protection as he begins his legal battle against a bid by the US to extradite him from Britain.

The Australian has decided to formally contest an American extradition request over a charge of conspiring to commit computer intrusion with former US Army intelligence analyst Chelsea Manning between January and May, 2010.

The 47-year-old on Thursday told Westminster Magistrates Court, which was so packed with press that some reporters had to sit on the floor, that he would contest the US application.

"I do not wish to surrender myself to extradition for doing journalism that has won many, many awards and protected many people," Assange told Judge Michael Snow via videolink from Belmarsh prison.

On Wednesday, Assange was sentenced to 50 weeks in prison for breaching his British bail.

Outside court his barrister Jennifer Robinson made a fresh appeal for the Australian government to intervene in the case.

"What we would like to see is action at the higher political levels and diplomatic protection being exercised over Julian Assange," Ms Robinson told AAP.

"We have been asking the Australian government since 2010 to seek assurances to protect him from US extradition, the very matter that he's facing right now and we would like the Australian government to be raising his case."

US government legal representatives Ben Brandon earlier told the court that Assange and Manning had made a clandestine deal to try to hack the passwords of Pentagon computers to access classified information.

"The charge relates to one of the largest compromises of classified information in the history of the United States," he said.

"Evidence obtained in the course of the investigation shows Ms Manning and Mr Assange unlawfully conspired to effectuate this disclosure."

Ms Robinson said afterwards there was no suggestion her client had ever hacked US military computers although she's concerned the Americans will add fresh charges to their extradition request.

She said the current case boiled down to allegations that Assange had communications with his source Manning, encouraged her to provide more information, and conversed with her about protecting her identity.

"Journalists do this all the time, that's why this indictment and the extradition request is such a concern for free speech groups here (in the UK) and in the US, and it's why Julian is committed to defending himself and resisting extradition in this case," Ms Robinson told AAP.

After the hearing, Assange's supporters abused media representatives leaving the courtroom.

One woman repeatedly screamed "Nazi racist scum" at journalists who were filing out past her while another ordered them to "do your jobs properly".

Outside the building, about 100 supporters waved banners and placards while chanting "Free, free Julian Assange" and "there's only one decision, no extradition". They also blocked the road.

One protest organiser told the crowd that Assange was being tried "in a secret court" because members of the public had been denied entry.

Lauri Love, a friend of Assange who himself successfully fought a US extradition request over alleged hacking, wheeled around a speaker playing protest songs in the background.

Assange was remanded in custody until another procedural hearing in the same court on May 30.

The extradition case is set to begin on June 12, with Mr Brandon successfully applying for one hour of proceedings that day.