Speaking outside court, WikiLeaks editor-in-chief Kristinn Hrafnsson said the sentence was an “outrage” and “vindictive in nature”.
“It doesn’t give us a lot of faith in the UK justice system for the fight ahead,” he told a crowd packed with journalists and supporters.
“Only two weeks short of the maximum is an outrage.”
Mr Hrafnsson said the extradition process is now the “big fight”.
“It will be a question of life and death for Mr Assange,” he said.
“It’s also a question of life and death for a major journalist principle.”
WikiLeaks founder Julian Assange has been jailed for 50 weeks at a UK court for a bail breach offence.
As he was taken down to the cells after being jailed for 50 weeks for breach of bail offences, Julian Assange defiantly raised his fist to the supporters in the public gallery behind him.
They raised their fists back at him in solidarity and shouted “Shame on you” towards the court in Southwark.
Announcing the sentence, Judge Deborah Taylor told Assange: “It’s difficult to envisage a more serious example of this offence.”
She added: “By hiding in the embassy you deliberately put yourself out of reach, while remaining in the UK.”
She told Assange that by doing so he had “exploited your privileged position to flout the law”.
Supporters have gathered outside the court in front of a large press pack.
Assange’s backers held aloft placards and banners demanding he be freed.
They chanted “Defend Julian Assange”, and “No extradition, there’s only one decision”.
Earlier: Julian Assange faces jail for breaching bail in UK
WikiLeaks founder Julian Assange is facing up to a year in an English jail for breaching his bail.
He was found guilty of breaching the Bail Act at a hearing at Westminster Magistrates' Court in April after being arrested at the Ecuadorian Embassy.
The Australian 47-year-old entered the dock today at a packed Southwark Crown Court wearing a dark jacket, light grey jumper and with a trimmed beard.
Some supporters stood as he came into courtroom one.
Asked if he understood that he had been committed for sentencing, he said: "I understand I have been committed. I don't know details."
A separate courtroom has been cleared for supporters and press in the building.
In mitigation for Assange Mark Summers QC, told the court his client had been "gripped" by fears of rendition to the US over the years because of his work with WikiLeaks.
He said: "As threats rained down on him from America, they overshadowed everything as far as he was concerned.
"They dominated his thoughts. They were not invented by him, they were gripping him throughout."
Mr Summers said Assange’s fears that he could face rendition from Sweden to the US were well founded and “not a figment of his imagination”.
Sweden at the time, he said, had a “well documented and unfortunate history” of sending “people to states where they were at significant risk of ill-treatment including torture and death”.
There were reports of discussions between Sweden and the US over the matter, Mr Summers said.
“That’s not a figment of his imagination,” he added.
“They were reasonable fears.”
In a letter read to court, Assange said: “I apologise unreservedly to those who consider that I have disrespected them by the way I pursued my case.
“I found myself struggling with difficult circumstances.
“I did what I thought at the time was the best or perhaps the only thing that I could have done.
“I regret the course that that has taken.”
Those difficulties, the letter continued, were “compounded” and also “impacted upon very many others”.
Assange was accused of sexual offences in Sweden in 2010, and after exhausting his legal options against an extradition order, went to the Ecuadorian Embassy on June 19, 2012.
A warrant for his arrest was issued 10 days later.
On Thursday, he will face a hearing about his potential extradition to the US over the allegation he conspired with intelligence analyst Chelsea Manning to infiltrate Pentagon computers.
Prosecutors in Sweden are also mulling whether to reopen the sexual assault case against Assange, which was dropped in May 2017. Assange denies the allegations.
At the hearing on April 11, District Judge Michael Snow remanded Assange in custody and branded him a "narcissist who cannot get beyond his own selfish interests".
The judge said: "This is a case which merits the maximum sentence, which is 12 months in the Crown Court."