Julian Assange’s lawyers say they have evidence the US considered “extreme” measures including poisoning and kidnapping the WikiLeaks founder after the publication of secret military files.

It was among a series of revelations in the first day of the Australian’s extradition hearing in London where a huge crowd of supporters including celebrities and human rights groups gathered, shouting “free Julian Assange” – the noise so loud that Assange struggled to hear the claims being made against him.

US prosecutors argued the Australian should be considered a “straightforward” criminal who put lives at risk and helped enemies like Osama bin Laden – not a protector of free-speech.

A sketch of Julian Assange sitting in the docks in a London court on Monday (UK time). Photo:AAP

The US Justice Department says it charged Assange with conspiring with former US military intelligence analyst Chelsea Manning to leak a trove of classified material in 2010.

The 48-year-old is facing 17 charges of violating the US Espionage Act and one of conspiring to commit computer intrusion over the leaking and publication of thousands of classified US diplomatic and military files.

Representing the US, James Lewis QC told Woolwich Crown Court WikiLeaks documents were found in Osama bin Laden’s compound after the al Qaeda leader was killed in Pakistan in 2011.

Mr Lewis said hundreds of sources had to be warned their safety was at risk after WikiLeaks dumped the secret files. He claimed that while some human sources were moved to safety others “subsequently disappeared” though he conceded the US could not prove it was a result of the leaks.

“What Mr. Assange seems to defend by freedom of speech is not the publication of the classified materials but the publication of the names of the sources, the names of people who had put themselves at risk to assist the United States and its allies,” Mr Lewis said.

Assange, his lawyers and his supporters have opposed his transfer to the US since his arrest in April last year.

They claim he would not get a fair trial and could face a prison sentence of up to 175 years if convicted.

Barrister Edward Fitzgerald was asked by District Judge Vanessa Baraitser if Assange would testify during the month-long hearing.

“Madam, it’s very unlikely that he would,” Mr Fitzgerald said.

Arguing against extradition, he said the dividing line between US courts and the president has been “blurred” in the decade-long pursuit of his client.

Mr Fitzgerald told the packed court a witness will confirm the US had even contemplated more “extreme measures” against the Australian.

“Such as kidnapping or poisoning Julian Assange in the embassy,” he said, referring to Assange’s seven-year asylum in the Ecuadorian embassy in London.

Mr Fitzgerald detailed how the Americans had spied on his meetings with lawyers in the embassy. He also maintained that US congressman Dana Rohrabacher had indeed offered Assange a pardon on orders of President Donald Trump, a claim both American men denied last week.

“President Trump denies everything and we say ‘well, he would, wouldn’t he’,” Mr Fitzgerald said.

Outside court, journalists and human rights group said the case could have wide-ranging impacts for media freedoms and whistleblowers.

Australian human rights lawyer Jennifer Robinson speaks outside Woolwich Crown Court in southeast London. Photo: Getty

“Today journalism itself is on trial,” said the Defend Assange Campaign, which has organised protests on Monday in cities including Berlin, Brussels, Vienna, Athens, New York, Washington, Sydney, Seoul and Mexico City.

Paris-based Reporters sans Frontieres (RSF) also called for Assange’s release and said it has sent monitors to the hearing in London.

“It is an unprecedented step for us to undertake this type of international trial-monitoring mission in an established democratic country, which would be more typical in countries ranked much further down our World Press Freedom Index,” the head of RSF’s London bureau Rebecca Vincent said.

The Council of Europe’s human rights commissioner Dunja Mijatovic has also urged Britain not to allow Assange’s extradition, saying the case raises important concerns about press freedom and the protection of the rights of whistleblowers.

The UN rapporteur on torture Nils Melzer has found Assange displayed “all the symptoms typical for prolonged exposure to psychological torture” after visiting him in prison in May.

London’s Metropolitan Police arrested Assange again in April at the Ecuadorian embassy for failing to surrender to an earlier warrant issued by a British court in relation to a Swedish extradition request.

A court sentenced Assange to 50 weeks in prison in May for breach of bail conditions linked to the Swedish charges of sexual assault, which were later dropped.

-with AAP