Actress Vanessa Redgrave has delivered a petition to Downing Street demanding the return of British prisoners at Guantanamo Bay in Cuba.
Moazzam Begg has been detained for more than two years
Azmat Begg, the father of one of the British detainees at the US camp, alleges they are being tortured.
Mr Begg's son, Moazzam, has been in American custody since his arrest in Islamabad, Pakistan, in February 2002.
Prisoner Richard Belmar's sister Janette, and Martin Mubanga's sister Katherine also handed in the forms.
Together with Feroz Abbasi, 23, from Croydon, south London, the men are among more than 600 foreigners held at the Cuban base.
The relatives are delivering a 3,000-signature strong petition to the minister of state, Baroness Symons.
It calls for the government to investigate allegations of torture during interrogations and to demand to see all recordings of violence.
Vanessa Redgrave was among a delegation that lobbied outside the US Supreme Court in Washington DC last March, denouncing treatment of inmates at the Cuban facility.
She said the British Government should repatriate the Guantanamo detainees and added: "Intensive interrogation techniques are the 'newspeak' for torture.
"Since the International Red Cross has only been allowed restricted access in this situation, I have to say that Guantanamo Bay is not a detention centre, it is a
Vanessa Redgrave has long campaigned with Azmat Begg
High profile religious leaders including the Right Reverend Richard Harries, Bishop of Oxford, Rabbi David Goldberg and Massoud Shadjareh, Chair of the Islamic Human Rights Commission, also joined the protest.
The Bishop of Oxford said: "It is simply unacceptable for prisoners of any nationality to be imprisoned without charge, without access to family visits, and without hope of a hearing, under a regime that treats them little better than animals. "
Azmat Begg says his son told in letters how he was kept in a cage at the centre, unable to move his hands or feet and denied food, clothing and light.
Abu Ghraib fears
Moazzam Begg, a father-of-four, from Birmingham, has been described as a "high risk" detainee but campaigners say the law student was in Afghanistan to teach children, rather than to fight American and British forces.
Mr Begg told BBC News US and Pakistani Army officers had "kidnapped" his son in Pakistan without giving a reason.
Mr Begg said revelations of the abuse of prisoners at Abu Ghraib jail in Iraq had fuelled his fears for his son's safety as US Army personnel there had previously been stationed at Guantanamo Bay.
"They should immediately bring him back to the country he belongs to," Mr Begg said.
"If he has done something, he should be punished. If he hasn't done anything wrong, he should be set free."