A UK judge was given two conflicting portraits of Julian Assange as the WikiLeaks founder's long-awaited extradition hearing began on Monday in a London court.

A lawyer for the US authorities, who want to try Assange on espionage charges, said the Australian computer expert was an "ordinary" criminal whose publication of hundreds of thousands of secret military documents a decade ago put many people at risk of torture and death.

"Reporting or journalism is not an excuse for criminal activities or a licence to break ordinary criminal laws," said James Lewis, a British lawyer representing the US government.

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Assange's lawyer countered that the WikiLeaks publisher was being victimised by a "lawless" US government that wanted to make an example of him.

Lawyer Edward Fitzgerald also said the "inhuman" conditions Assange was likely to face in an American prison would put him at high risk of suicide.

Dozens of Assange supporters protested outside the high-security Woolwich Crown Court, chanting and setting off a horn as District Judge Vanessa Baraitser began hearing the case, which is due to last several months.

Assange, 48, watched proceedings from the dock in the courtroom - he was brought there from Belmarsh Prison next door.

He complained that he was having difficulty concentrating and called the noise from outside "not helpful."

Assange has been indicted in the US on 18 charges over the publication of classified documents.

Prosecutors say he conspired with US army intelligence analyst Chelsea Manning to crack a password, hack into a Pentagon computer and release secret diplomatic cables and military files on the wars in Iraq and Afghanistan.

Assange argues he was acting as a journalist entitled to First Amendment protection, and says the leaked documents exposed US military wrongdoing.

Among the files published by WikiLeaks was video of a 2007 Apache helicopter attack by American forces in Baghdad that killed 11 people, including two Reuters journalists.

But Lewis said Assange was guilty of "straightforward" criminal activity in trying to hack the computer.

He said WikiLeaks' activities created a "grave and imminent risk" to US intelligence sources in Afghanistan and Iraq.

"By disseminating the materials in an unredacted form, he likely put people - human rights activists, journalists, advocates, religious leaders, dissidents and their families - at risk of serious harm, torture or even death," the lawyer said.

Assange's lawyers argued that the charges - which carry a maximum sentence of 175 years in prison - are a politically motivated abuse of power.

Fitzgerald said Assange was suffering as a result of the "declaration of war on leakers and journalists" by President Donald Trump.

"Julian Assange has been made an example of," Fitzgerald said. "He was the obvious symbol of all that Trump condemned."

Assange's legal team also alleges that the Australian was offered a pardon by the administration of US President Donald Trump if he agreed to say Russia was not involved in leaking Democratic National Committee emails that were published by WikiLeaks during the 2016 US election campaign.

Assange's lawyers say the offer was made in August 2017 by then-Republican Congressman Dana Rohrabacher, who claimed to be acting with the approval of Trump.

The White House has called the claim a fabrication.

Assange's legal saga began in 2010, when he was arrested in London at the request of Sweden, which wanted to question him about allegations of rape and sexual assault made by two women. He refused to go to Stockholm, saying he feared extradition or illegal rendition to the United States or the US prison camp at Guantanamo Bay, Cuba.

In 2012, Assange sought refuge inside the Ecuadorian embassy, where he was beyond the reach of UK and Swedish authorities.

The relationship between Assange and his hosts eventually soured and he was evicted in April 2019. British police immediately arrested him for jumping bail in 2012.

Sweden dropped the sex crimes investigations in November, but Assange remains in prison as he awaits a decision on the US extradition request.

As the hearing got under way on Monday, Nils Melzer, UN special rapporteur on torture, tweeted: "This is not just about Julian Assange. This is a battle over press freedom, the rule of law, and the future even of democracy. For democracy cannot co-exist with secrecy. When you deprive the public of their right to know, you deprive them of the tools to control their government."

After a week of opening arguments, the extradition case is due to break until May, when the two sides will lay out their evidence. The judge is not expected to rule until several months after that, with the losing side likely to appeal.