Belmarsh jail has frequently been used in high-profile national security cases, earning the moniker "the UK's Guantanamo Bay" in the period following the 9/11 attacks after it was used to detain a number of people.
German and Spanish lawmakers protested against Julian Assange's detention on Monday, gathering outside a London prison to urge Britain and the EU to prevent his extradition to the United States.
Two leftist German MPs, Heike Hansel and Sevim Dagdelen of Die Linke, and a Spanish Green member of the European Parliament, Ana Miranda, had been due to meet their "friend" Assange in London's Ecuadoran embassy later on Monday.
Instead, following his expulsion and arrest last week, they protested outside the top-security Belmarsh prison in southeast London where he is being held, carrying placards demanding his release.
"We are faced with a humanitarian imperative now that Assange is in UK custody and a U.S. extradition request is out for him, after high ranking officials of the United States - including President Donald Trump - have threatened the publisher with death," said Miranda.
The WikiLeaks founder is in custody awaiting sentencing for breaching his British bail conditions in 2012 by seeking refuge in Ecuador's London embassy to avoid extradition to Sweden.
He was arrested at the embassy on Thursday after Ecuador revoked his asylum, and is now also fighting a U.S. extradition warrant relating to the release by WikiLeaks of a huge cache of official documents.
The U.S. indictment charges Assange with "conspiracy" for working with former U.S. Army intelligence analyst Chelsea Manning to crack a password stored on Department of Defense computers in March 2010.
Extradition to the United States would set "a dangerous precedent" for journalists worldwide, said Miranda, adding: "this threatens to criminalise journalism globally."
"We call on the British government not to extradite Julian Assange to the United States. The European Union must take action to protect a politically-persecuted publisher and journalist," said the German politician, calling on Spain and her home country to grant Assange asylum.
The three women were trying to see Assange in prison, but their application was "still pending."
"I'm a friend of Julian Assange. He's a son, a father, a brother and a good friend," said Dagdelen. "His whole life he sacrificed for the truth."
They also accused the Ecuadoran government of "engaging in a disinformation and slander campaign against Assange" after a series of reports emerged detailing his increasingly disfunctional relationship with embassy staff.