The UN said more than 1,000 may have been detained, and the whereabouts of 50 of them are unknown [Gallo/Getty]
The United Nations human rights chief has called for Bahrain to free activists it has seized since crushing anti-government protests and for an independent probe into allegations of torture.
UN High Commissioner for Human Rights Navi Pillay's remarks on Thursday were the sharpest international criticism yet of the crackdown in Bahrain where the Sunni-led government has arrested hundreds from its Shia Muslim majority since the protests began.
"All political detainees must be immediately released and all detainees must have prompt access to legal counsel," Pillay said in a statement.
"My office has also received reports of severe torture against human rights defenders who are currently in
detention... There must be independent investigations of these cases of death in detention and allegations of torture."
At least four people have died in detention, and rights groups have criticised death sentences handed out last week to four men accused of killing policemen in March during protests that began with calls for more political liberties in the kingdom.
The defendants in that case were accused of running down two policemen with a car in March.
Also on Thursday, one man was sentenced to at least five years in jail and another was acquitted. State media earlier said they were accused of trying to kill security personnel by running them over.
At least 13 protesters and four police died in the unrest. Bahrain has said about 400 of those detained will face prosecution, some in a military court that last week handed down the first death sentence to a Bahraini citizen since the mid-1990s, also a period of sectarian-tinged political turmoil.
'No legal defence'
Families of the condemned men and rights groups say the court, which also sentenced three men to life in prison, kept the defendants from meeting lawyers and mounting any defence.
"The application of the death penalty without due process and after a trial held in secrecy is illegal and absolutely unacepptable," Pillay said.
The UN statement said more than 1,000 may have been detained, and the whereabouts of 50 of them are unknown.
New York-based Human Rights Watch said in a statement on Wednesday that it had received credible reports that a detained human rights activist, Abdulhadi al-Khawaja, had been hospitalised following beatings while in custody.
Amnesty International also appealed on Wednesday to Bahrain's rulers to bring an end to a campaign of arrests against its opponents while denouncing the extension of emergency rule in response to protests.
"The Bahraini authorities must stop detaining anyone who opposes them and release protesters who have been locked up for peacefully demanding reform," said Philip Luther, Amnesty's deputy director for the Middle East and North Africa.
"Even since the protests on the streets were violently crushed in mid-March the government's persecution of dissidents has not abated, while the renewal of the so-called 'State of National Safety' will only exacerbate this human rights crisis."