Lawyers for Julian Assange have accused the US of "state retaliation" over its bid to prosecute the Wikileaks founder.
Mr Assange has been in Belmarsh - a UK prison - since 2019, and is wanted by the American authorities for disclosing secret military files in 2010 and 2011.
At a two-day High Court hearing, which began on Tuesday, his legal team argued it would be against UK law to hand him over.
If an appeal is turned down, Mr Assange could be extradited within weeks.
Edward Fitzgerald KC, one of the 52-year-old Australian's lawyers, argued that the US's prosecution bid was "politically motivated".
"Mr Assange was exposing serious criminality" when he disclosed the documents in question, Mr Fitzgerald told judges Dame Victoria Sharp and Mr Justice Johnson.
He told them his client was "being prosecuted for engaging in [the] ordinary journalistic practice of obtaining and publishing classified information - information that is both true and of obvious and important public interest."
Another of Mr Assange's lawyers, Mark Summers KC, said the US sought retribution for Mr Assange's political opinions - one of many bars to extradition from the UK, as set out by the Crown Prosecution Service (CPS).
"This is a paradigm example of state retaliation for the expression of political opinion," Mr Summers told the court in central London.
The lawyers also argued that their client was at "a real risk of further extrajudicial actions ... by the CIA [Central Intelligence Agency] or other agencies" - a legally delicate way of saying he could be assassinated or subject to some harm beyond a criminal sanction after a fair trial.
Their allegation - not yet evidentially tested - is that the CIA plotted to kill Mr Assange during the seven years he took refuge inside Ecuador's London embassy, from 2012 to 2019.
Mr Summers told the judges that then-US President Donald Trump had asked for "detailed options" of how to kill Mr Assange, who was not present in court on Tuesday due to sickness.
"Sketches were even drawn up," he said, adding there was evidence of this "truly breath-taking plan" - though none has so far been produced.
Mr Summers said the alleged plan "only fell apart when the UK authorities weren't very keen on the thought of rendition, or a shootout, in the streets of London".
In written submissions, he and Mr Fitzgerald added: "The evidence showed that the US was prepared to go to any lengths, including misusing its own criminal justice system, to sustain impunity for US officials in respect of the torture/war crimes committed in its infamous 'war on terror', and to suppress those actors and courts willing and prepared to try to bring those crimes to account.
"Mr Assange was one of those targets."
Mr Assange's mammoth legal battle began in 2010, when Wikileaks disclosed huge numbers of confidential military files from the wars in Iraq and Afghanistan - including footage showing a US helicopter gunning down civilians in Baghdad.
He took refuge in London's Ecuadorian embassy, before being arrested by the Metropolitan Police in 2019.
The US demanded his extradition from the UK that year, saying the disclosures endangered lives.
Two years later, a British judge ruled that while the US had shown it had a legitimate criminal case against Mr Assange, he could not be transferred because he may try to harm himself.
The US later overturned that ruling after giving the UK new assurances about how Mr Assange would be treated in the event of extradition, including the possibility of spending any possible jail sentence in his native Australia.
At this week's hearing, largely seen as a last-ditch attempt, Mr Assange's lawyers are asking for permission to challenge the extradition order signed by the then UK home secretary Priti Patel in 2022.
If they fail to convince judges there is anything wrong with that order, Mr Assange must be extradited within 28 days - unless he can convince the European Court of Human Rights to temporarily stop the flight with a so-called "Rule 39" order.
Nick Vamos, the former head of extradition at the Crown Prosecution Service, said US Marshals could arrive in London within days if the High Court throws the case out.
"There is a very high threshold for [the European Court of Human Rights to intervene], namely that there is 'an imminent risk of irreparable damage' to his human rights, which of course is one of the arguments the High Court in London would have just rejected," he said.
Case 'will determine if he lives or dies'
Speaking to the BBC on Monday, Stella Assange said her husband would not survive in a US jail - and described the case as politically-motivated.
"This case will determine if he lives or dies," she said.
Outside the High Court on Tuesday, supporters of Mr Assange gathered, waving placards featuring the words "Free Julian Assange".
Mrs Assange thanked them for their backing and, addressing them from a stage outside court, said: "We have two big days ahead. We don't know what to expect, but you're here because the world is watching."
Speaking to the BBC, she described her husband as the "victim" of US "retaliation", echoing the words of Mr Assange's lawyers inside.