It is feared that thousands of Iraqis are being held in secret prisons, making a mockery of Saddam Hussein's promise of a "comprehensive and final amnesty" for all prisoners.
"No authoritarian regime like his would survive without secret prisons," a senior UN diplomat in New York said.
Amnesty International last week welcomed the recent releases but said that there were "worrying reports" of dozens of secret prisons in Iraq, possibly as many as 70.
Neil Durkin, a spokesman for the group, says: "Given the fact that tens of thousands of people arrested in the past were, until recently, held without charge or trial, it is highly likely that very large numbers of detainees are still held in secret by the Iraqi authorities."
Details of some of the secret prisons have leaked from those who have escaped and from relatives who paid bribes to visit inmates, dissidents say.
By emptying its official jails, Baghdad hoped to improve its image abroad and rally support at home in a desperate attempt to ease growing American and British pressure. Scores of jubilant former inmates were filmed singing Hussein's praises as they spilled out of Baghdad's Abu Ghraib prison, the country's biggest and most infamous jail. Most are thought to have been common criminals rather than political prisoners.
Built for 2,000, Abu Ghraib was crammed with an estimated 10,000 inmates living in appalling conditions. Death-row prisoners languished in overcrowded and unlit cells with the only light coming from a dim bulb dangling in the corridor.
Mr. Hussein's public relations gesture backfired in following days when several hundred people dared to demonstrate in Baghdad to demand information on family members who were arrested several years ago and were not released.
"We'd have to see solid evidence of the release of political prisoners before this amnesty can be taken seriously. There is a strong air of theatricality about it," says Prof. Toby Dodge, an Iraq expert at Warwick University in Britain.
Dissidents interviewed for this story say there may be few genuine political prisoners left in Iraqi jails, suspecting that any seriously opposed to Saddam have been executed. Others classed as political prisoners by the regime may simply be ordinary people who have been overheard criticizing Saddam and are not part of any organized opposition, dissidents add.
"These guys are jailed to put the fear of God into everybody," says Saad Jabr, the leader of the opposition Free Iraqi Council. Most of those being held secretly are from Iraq's Shiite majority. Ayatollah Muhammad Baqir al Hakim, the leader of the Supreme Council of the Islamic Revolution in Iraq, the main Shiite opposition group, said that thousands of "academic and social figures" had not been released while the fate of tens of thousands of others remained unknown.
"Prisoners are liquidated all the time, without informing their families," he told a Gulf newspaper.
Relatives live in a limbo of uncertainty, unsure whether those detained are alive or dead. Thousands of prisoners have been executed during Hussein's 23 years in power.
"In Iraq, the routine practice of holding detainees in incommunicado detention for months or even years has meant that information about their fate has been incredibly difficult to obtain," Durkin says.
Amnesty sent a letter to the Iraqi government immediately after the prisoner releases were declared to seek urgent clarification of the fate of thousands who disappeared in the 1980s and after the 1991 Gulf War.
These include more than 600 Kuwaiti and other nationals, as well as 106 Shiite clerics and students who were arrested in the southern city of al Najaf, Amnesty said.
Secret prisons have been discovered in the past during times of turmoil. When Shiite rebels seized the southern city of Basra in 1991, they uncovered a secret underground prison opposite the mayor's office.
"Some of the hundreds of prisoners had been shut off from the world for so long that they shouted 'Down with al Bakr' as they were released," according to "Out of the Ashes," a book on Iraq by Andrew Cockburn and Patrick Cockburn.
The freed inmates were under the impression that the president of Iraq was still Ahmed Hassan al-Bakr, who had been replaced by Hussein in 1979.
According to "Out of the Ashes," Similar horrors were uncovered by rebellious Kurds in northern Iraq. The corpses of recently strangled women and children were found in a stone fortress used by Hussein's security apparatus in Sulaimaniya.
"As in Basra, some of the prisoners had been sealed in underground cells for more than a decade," according to the book.
While there is deep skepticism over Saddam's amnesty, human rights activists welcomed the gesture even if it brought only short-lived respite for prisoners.
One said: "Even half a day's freedom from Abu Ghraib is a blessing."