Human rights groups have called on the US government to allow the UN free access to Guantanamo detainees.
Ex-Guantanamo inmate Moazzam Begg will speak in London
Amnesty International and Reprieve made the call as they launched a three-day conference in London on human rights, prevention of torture and detentions.
Amnesty said the event would be the largest gathering of former Guantanamo detainees and their families held yet.
The UN has rejected a US invitation to visit Guantanamo because of the restrictions imposed by Washington.
UN officials have been trying to visit the camp in Cuba since it opened in January 2002.
But a deadline passed at midnight on Friday without the US and UN human rights monitors reaching agreement on conditions for an independent inspection.
Human rights campaigners have expressed growing concern about the treatment of the inmates at Guantanamo, a number of whom are on hunger strike.
The US is also under increasing domestic and international pressure to answer allegations that the CIA is operating secret prisons abroad.
Some detainees have been held for nearly four years without charge
Speaking in London, Amnesty Secretary General Irene Khan said: "Denying meaningful access to those held in Guantanamo Bay is totally unacceptable.
"Through the courageous testimonies of former prisoners and prisoners' families, our conference this weekend will highlight how Guantanamo has become the epicentre of a shadow justice system."
Among those taking part is Moazzam Begg, one of nine British nationals detained at Guantanamo Bay to have been released without charge.
Former detainees from Russia and Afghanistan are also expected to speak at the conference.
Clive Stafford Smith, legal director for Reprieve and lawyer for some 40 Guantanamo Bay detainees, called on the UK government to act on behalf of British inmates taking part in the hunger strike.
About 500 prisoners remain at Guantanamo, many of them captured in Afghanistan. Some have been held for nearly four years without charge.
The Pentagon has said only the Red Cross needs free access to prisoners.