Two far-left 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.
"We are faced with a humanitarian imperative now that Assange is in UK custody and a US extradition request is out for him, after high ranking officials of the US -- including President Donald Trump -- have threatened the publisher with death," said Miranda.
He was arrested at the embassy on Thursday after Ecuador revoked his asylum, and is now also fighting a US extradition warrant relating to the release by WikiLeaks of a huge cache of official documents.
Extradition to the United States would set "a dangerous precedent" for journalists worldwide, said Miranda, adding: "this threatens to criminalise journalism globally."
Dagdelen urged Britain and the EU to block any extradition request.
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.
Belmarsh has frequently been used in high-profile national security cases, including that of former Finsbury Park Mosque hate preacher Abu Hamza, who now resides in a US "supermax" prison following extradition.
Radical cleric Anjem Choudary, who was convicted in 2016 of encouraging support for the Islamic State group, spent some of his sentence there.
Belmarsh earned 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 under anti-terror laws.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)