A lawyer for Julian Assange on Monday claimed the United States' bid to extradite him is part of President Donald Trump's "war on leakers and journalists", as the WikiLeaks founder's full extradition hearing opened in Britain.

Assange faces charges under the US Espionage Act for the 2010 release by his anti-secrecy website of a trove of files detailing the realities of US military campaigns in Afghanistan and Iraq.

He spent much of the past decade holed up in Ecuador's London embassy to avoid separate legal proceedings in Sweden, but Washington is now seeking his transfer to stand trial.

His lawyer Edward Fitzgerald argued the charges were "politically motivated" and that the US had reversed a 2013 decision not to charge Assange because Trump wanted "to make an example" of him.

"President Trump came into power with a new approach to the freedom of the press... amounting effectively to declaring war on investigative journalists," he told a packed courtroom in southeast London.

Fitzgerald said his client was "the obvious symbol of all that Trump condemned" and that charging him for publishing state secrets was "unprecedented".

"The prosecution is being pursued for ulterior political motives and not in good faith," he added. "It is directed at him because of the political opinions he holds and that have guided his actions."

Assange sat impassively inside Woolwich Crown Court as a lawyer for the United States accused him of risking the lives of intelligence sources by publishing classified US government documents.

Making the US government case, James Lewis denied it was motivated by embarrassment over the WikiLeaks releases and wanted him over the "harm" caused by his disclosures.

"The United States is aware of sources whose unredacted names and/or other identifying information was contained in classified documents published by Wikileaks who subsequently disappeared," he said.

However, he noted the United States could not prove that their disappearances were directly linked to WikiLeaks.

"Julian Assange is no journalist," Lewis added.

Protesters

Dozens of protesters gathered throughout the day outside the court, next to the high-security Belmarsh prison where Assange is being held, holding up banners and chanting loudly.

At one point during the proceedings Assange, wearing a dark grey blazer and sweater over a white shirt and flanked by two security personnel, complained he was having "difficulty concentrating" due to the demonstrators' noise.

"I'm very appreciative of the public support," he said. "I do understand that they must be disgusted by these proceedings."

WikiLeaks initially worked with a string of high-profile newspapers to publish details from the leaked State Department and Pentagon files, which caused a sensation -- and outrage in Washington.

One video from 2007 showed an Apache helicopter attack in which US soldiers gunned down two Reuters reporters and nine Iraqi civilians in broad daylight in Baghdad.

But after falling out with their editors, WikiLeaks released hundreds of thousands of documents in their original form -- including the secret identities of diplomats and local sources.

Assange, 48, could be jailed for 175 years if convicted on all 17 Espionage Act charges and one count of computer hacking that he faces.

It is the most serious phase of a long-running legal saga.

In 2010, Assange was accused of sexual assault and rape in Sweden, allegations he consistently denied. After a legal battle, was ordered by a British judge to be extradited there.

To avoid extradition, he claimed asylum in Ecuador's London embassy, where he spent seven years until Quito gave him up last year following a change of government.

The Swedish investigation has since been dropped.

Assange has since served a jail term for breaching his UK police bail but remains in Belmarsh prison awaiting the US extradition case.

Trump claim

In a new twist last week, Assange's defence team claimed Trump promised to issue a pardon if Assange denied Russia leaked the emails of his 2016 election rival's campaign -- although Assange has always denied that Russia had any part in the affair.

The White House called the claim "another never-ending hoax and total lie" but a judge agreed to add it to the case file.

In his courtroom remarks Monday, Fitzgerald said the US had used "the threat of prosecution as extortion".

Noting Trump's denial of a pre-emptive pardon offer, he added: "Well he would, wouldn't he?"

Fitzgerald also detailed the testimony of a whistleblower -- "witness two" -- in Spain who worked at the company providing private security at the Ecuadoran embassy in London and allegedly set up a spying operation on Assange for the United States.

The lawyer said he plans to apply for the whistleblower's evidence to be heard.