Hundreds of Julian Assange’s supporters from across Europe gathered in London on Saturday to demand that the WikiLeaks founder be released from detention and spared extradition to the US.

Italians and Germans were among those showing their support for the 48-year-old before his extradition hearing opens at Woolwich crown court on 24 February. Assange’s father John Shipton addressed the crowd in Parliament Square. The protesters brandished banners with slogans such as “Journalism is not a crime”.

The United States wants Assange to face 18 charges over the publication of classified government documents, which could result in a 170-year prison sentence.

Shipton told the protesters that he did not understand why his son was being held in Belmarsh prison, in south-east London. “I bring to you his affection, his nobility of purpose and his strength of character after nine years,” he said.

Almost a decade has passed since WikiLeaks published secret US diplomatic cables and documents about the wars in Iraq and Afghanistan, which Assange’s supporters say shed crucial light on American abuses. Kristinn Hrafnsson, editor in chief of WikiLeaks, told the protesters that they were standing against a “dark force”. He said: “This is not about left or right, we can unite on this, it is a dark force against [those] who want justice, transparency and truth.”

Other speakers included the former Greek finance minister Yanis Varoufakis, Pink Floyd’s Roger Waters and the fashion designer Vivienne Westwood, who wore a halo with Assange’s name on it and referred to herself as “the angel of democracy”. His supporters claim the extradition attempt is politically motivated and driven by people who are embarrassed by WikiLeaks’s revelations.

Among the crowd was a 24-year-old wearing a gold face mask who had flown in from Berlin in the morning. She said she also wanted to make a statement against Boris Johnson. “Johnson wants to break all the laws, the rule of law. He is a very real threat for all of us,” she said.

Wolf Pozinski, 60, from Amsterdam, also wanted to show his support. He said: “It’s important that people like Assange are not criminalised for journalism that revealed a war crime.”

In 2010 WikiLeaks published a classified US military video showing a 2007 attack by Apache helicopters in Baghdad that killed a dozen people, including two Reuters news staff.

Two years later, Assange took refuge in Ecuador’s London embassy to avoid extradition to Sweden where he was accused of sex crimes. However, last November Swedish prosecutors said they were discontinuing an investigation into a rape allegation, explaining that although the complainant’s evidence was deemed credible and reliable, witnesses’ memories had faded over the decade since the allegations were first made. Assange has always denied the allegations.

He was removed from the embassy last April and was arrested for failing to surrender to the court. He has been in prison ever since after the US lodged its extradition request.