Julian Assange faces the risk of a “flagrant denial of justice” if tried in the US, his lawyers have told a permission to appeal hearing in London, which could result in the WikiLeaks founder being extradited within days if unsuccessful. Assange, who published thousands of classified military and diplomatic documents relating to the Afghanistan and Iraq wars, could be jailed for up to 175 years – “a grossly disproportionate punishment” – if convicted in the US, the high court heard on Tuesday. Edward Fitzgerald KC, representing Assange, also argued that his client could be targeted by US state agencies for “extra-legal attack elimination” if he was extradited, particularly given “the real possibility of a return of a [Donald] Trump administration”. Assange’s lawyers are seeking a full appeal hearing. If the two judges deny permission, all challenges in the UK courts will have been exhausted, leaving an intervention by the European court of human rights (ECHR) as Assange’s only hope to avoid extradition to the US. Outside the court, scores of his supporters held placards and chanted, demanding his release. The WikiLeaks founder was granted permission to attend the two-day hearing but Fitzgerald said Assange was unwell. Fitzgerald told the court that if Assange was extradited there was “a real risk that he’ll suffer a flagrant denial of justice”. In written arguments, Fitzgerald said: “This legally unprecedented prosecution seeks to criminalise the application of ordinary journalistic practices of obtaining and publishing true classified information of the most obvious and important public interest.” He said Assange and WikiLeaks “were responsible for the exposure of criminality on the part of the US government on an unprecedented scale”, including torture, rendition, extrajudicial killings and war crimes. One of the most infamous disclosures was video footage of a helicopter attack by US forces that killed 11 people in Iraq, including two Reuters journalists. Fitzgerald alleged the US prosecution was motivated by “state retaliation” and so was unlawful. Among the grounds on which Assange is seeking permission to appeal is the claim that his extradition is in breach of the extradition treaty between the UK and the US, which prohibits doing so for political offences. Assange faces 17 charges of espionage, which Fitzgerald said was manifestly a political offence and politically motivated, as well as one of computer misuse. “The prohibition on extradition for political offences, reflected in article 4 [of the extradition treaty], has venerable historic and juristic importance,” Fitzgerald told the court. “It is one of the most fundamental protections recognised in international and extradition law … Other western countries and governments stand firm against US extradition requests for ‘political offences’.” Mark Summers KC, also for Assange, raised the issue of a “breathtaking” plan for rendition or murder of the WikiLeaks founder while he was sheltering in the Ecuadorian embassy, which was reported by Yahoo News. Fitzgerald cited those allegations when arguing there was a real risk Assange “could be targeted by US state agencies as a ‘hostile, non-state actor’ meriting the application of clandestine and extra-legal attack or elimination”. Organisations backing Assange include Reporters Without Borders, PEN International, the National Union of Journalists, Amnesty International and Human Rights Watch. Outside the court, Assange’s wife, Stella, told the crowd: “We have two big days ahead, we don’t know what to expect, but you’re here because the world is watching. They just cannot get away with this. Julian needs his freedom and we all need the truth.” She told reporters her husband’s case was analogous to that of Alexei Navalny, the Russian opposition activist who died in prison on Friday. “Julian is a political prisoner and his life is at risk. What happened to Navalny can happen to Julian,” she said. If Assange is refused permission to appeal, he will have to apply to the ECHR to order the UK not to extradite him while it considers his case. If the application is refused he could be removed from the country by US marshals within days. The US will have the opportunity to make oral arguments on Wednesday but, in written arguments, it accused Assange’s lawyers of having “consistently and repeatedly misrepresented” the case. James Lewis KC said the WikiLeaks founder was not being prosecuted for “mere publication” but for “aiding and abetting” or “conspiring with” the whistleblower Chelsea Manning to unlawfully obtain the documents in question, “undoubtedly committing serious criminal offences in so doing and then disclosing the unredacted names of sources (thus putting those individuals at grave risk of harm)”.