Julian Assange is set to face off with the US government when his extradition hearing begins on Monday.

A judge at Woolwich crown court in south-east London is due to begin hearing arguments from lawyers for US authorities, who want to try the WikiLeaks founder on espionage charges that carry a maximum sentence of 175 years in prison.

After a week of opening arguments, the extradition case is scheduled to be adjourned until May, when the two sides will lay out their evidence. The judge is not expected to rule until several months after that, with the losing side likely to appeal.

If the courts approve extradition, the British government will have the final say.

Assange, 48, is wanted on 18 charges over the publication of US cables a decade ago. He is accused of working with former US army intelligence analyst Chelsea Manning to leak hundreds of thousands of classified documents.

US authorities say WikiLeaks’ activities put American lives in danger. Assange argues that he was acting as a journalist entitled to First Amendment protection, and says the leaked documents exposed US military wrongdoing.

Journalism organisations and civil liberties groups including Amnesty International and Reporters Without Borders say the charges set a chilling precedent for freedom of the press.

Assange’s father, John Shipton, who visited him in Belmarsh prison on Sunday, claimed his son was “harassed” by a prison cell search ahead of the hearing at the nearby court. Speaking to reporters outside, Shipton demanded Assange be released on bail.

“For the life of me I can’t understand why Julian Assange is in jail having committed no crime, with family here that he can come and live with,” he said. “Bail ought to be given immediately if the extradition order isn’t dropped.

Julian Assange’s father, John Shipton, and economist Yanis Varoufakis outside HMP Belmarsh. Photograph: Yui Mok/PA

“Julian had a harassment today. He goes to court tomorrow. They searched his cell this afternoon just before he came down to see us. This plague of malice that emanates from the Crown Prosecution Service to Julian Assange must stop immediately.”

Shipton was accompanied on his visit by Greek economist Yanis Varoufakis. They were met by representatives from Reporters Without Borders as they left. Varoufakis said Assange was in a “very dark place” due to spending more than 20 hours a day in solitary confinement.

Shipton said it was “distressing” and “very upsetting” to see his son in jail, adding that he had told him his whole family would be supporting him. Last week Shipton warned that his son’s extradition would be akin to a “death sentence”.

He addressed hundreds of Assange supporters after they marched through central London to Parliament Square on Saturday to protest against his son’s potential extradition.

On Sunday evening, Assange supporters were setting up tents outside the court building.