A struggle over the US request for Julian Assange’s extradition will open in court on Thursday morning, a day after the WikiLeaks founder was jailed for just under a year for breaching bail conditions to avoid being extradited to Sweden.

Wednesday’s sentence was decried as an “outrage” by Kristinn Hrafnsson, the editor-in-chief of the whistleblowing website, who said the hearing at Westminster magistrates court to oppose Assange’s extradition would be the start of the “big fight” – a process he said would be “a question of life and death for Mr Assange”.

A judge largely rejected the mitigating factors put forward by lawyers for Assange – who took refuge in Ecuador’s embassy to London in 2012 to avoid extradition to Sweden over sexual assault allegations, which he has denied – and told the 47-year-old it was difficult to envisage a more serious example of the offence.

“You remained there for nearly seven years, exploiting your privileged position to flout the law and advertise internationally your disdain for the law of this country,” said Judge Deborah Taylor, as she sentenced him at Southwark crown court.

WikiLeaks’ editor-in-chief Kristinn Hrafnsson (centre left) and barrister Jennifer Robinson (centre right) outside Southwark crown court. Photograph: Tolga Akmen/AFP/Getty Images

“Your actions undoubtedly affected the progress of the Swedish proceedings. Even though you did cooperate initially, it was not for you to decide the nature or extent of your cooperation with the investigations. They could not be effectively progressed, and were discontinued, not least because you remained in the embassy.”

Assange, who was arrested last month when Ecuador revoked his political asylum and invited Metropolitan police officers inside the country’s Knightsbridge diplomatic premises, had written a letter in which he expressed regret for his actions but claimed he had been left with no choice.

“I apologise unreservedly to those who consider that I have disrespected them by the way I have pursued my case. This is not what I wanted or intended,” he said in the letter read out by his lawyer, Mark Summers QC.

“I found myself struggling with terrifying circumstances for which neither I nor those from whom I sought advice could work out any remedy. I did what I thought at the time was the best and perhaps the only thing that could be done – which I hoped might lead to a legal resolution being reached between Ecuador and Sweden that would protect me from the worst of my fears.”

Assange, wearing a black blazer and shorn of the beard worn when police carried him out of the embassy last month, was told by the judge that his continued residence there had cost £16m of taxpayers’ money “in ensuring that when you did leave, you were brought to justice”.

“It is essential to the rule of law that nobody is above or beyond the reach of the law,” said the judge, who said Assange’s written apology was the first recognition that he regretted his actions.

The judge was met with cries of “shame on you” from Assange’s supporters in the gallery when she sentenced him to 50 weeks in prison – two weeks short of the maximum under guidelines – and directed that time spent on remand since his arrest on 11 April would count against it. Assange turned and raised a clenched fist to supporters as he was led away.

Assange after his arrest in April. Photograph: Henry Nicholls/Reuters

The WikiLeaks’ founder’s lawyer had earlier laid out a number of mitigating factors, claiming Assange lived in fear of being rendered from Sweden to the US, where politicians had talked of having him assassinated.

The case of Chelsea Manning and the conditions in which the US military whistleblower was kept was also mentioned, as was the case of individuals who were rendered from Sweden to the US in chains and after being drugged for transatlantic flights.

Summers told the court the Australian had been “gripped” by fears that his work with WikiLeaks would provoke rendition to Guantanamo Bay or the US, where he could face the death penalty.

“As threats rained down on him from America, they overshadowed everything as far as he was concerned,” he said. “They dominated his thoughts. They were not invented by him, they were gripping him throughout.”

He said the fact Assange chose indefinite detention in small rooms at the Ecuadorian embassy, in a state of depression and pain for various medical ailments, rather than spend 12 months in a British jail, showed the extent of his fears.

Speaking on the steps of the court afterwards, Hrafnsson said the sentence was vindictive, adding: ““It doesn’t give us a lot of faith in the UK justice system for the fight ahead.”

Drawing a comparison with the sentence of Jack Shepherd, who killed a woman in a speedboat crash and later fled to Georgia, he added: “And may I point out, just in comparison, that the so-called speedboat killer got six months for not showing up in court to hear his sentencing for manslaughter.”

Shepherd was given credit for his guilty plea, while Assange denied the bail offence and was convicted by a court.