The UN human rights chief has condemned the "shocking" use of force by security forces against protesters in Bahrain.Navi Pillay said reports of a military takeover of hospitals was a blatant violation of international law.
She urged the authorities to rein in their forces, citing reports of people being beaten and detained or killed.
At least six opposition figures have reportedly been detained in overnight raids in Bahrain, and soldiers are back on the streets of the capital, Manama.
The protesters, mostly from the majority Shia population, say the violence will not deter them from challenging Bahrain's Sunni rulers.
The strong UN reaction comes a day after a crackdown on anti-government protesters in the centre of the capital, Manama, that left at least three civilians and three police officers dead.
The streets are said to be calm, but extremely tense, with soldiers patrolling after an overnight curfew.
Correspondents say the opposition, which is seeking political reform, has gone to ground to plan its next move.
'Wrong track' Matar Ibrahim, a former opposition MP from Bahrain's Shia majority, who recently resigned in protest at the crackdown, told the BBC the government was completely ignoring the message from the US that it was on the wrong track.
The US State Department has also criticised the use of excessive force and Secretary of State Hillary Clinton has called for dialogue.
The BBC's Caroline Hawley, in Manama, says opposition attitudes have hardened since the crackdown.
Mr Ibrahim told the BBC: "We refuse to enter a dialogue while there are guns pointed at our heads."
Mid-East unrest: Bahrain
- King Hamad, 61, has been in power since 1999
- Population 800,000; land area 717 sq km, or 100 times smaller than Irish Republic
- A population with a median age of 30.4 years, and a literacy rate of 91%
- Youth unemployment at 19.6%
- Gross national income per head: $25,420 (World Bank 2009)
The arrested men include Hassan Mushaima and Abdeljalil al-Singace from the Haq party, who had been on trial for attempting to overthrow the leadership, although charges were dropped in a bid to calm tensions when unrest began.More moderate protesters are calling for a constitutional monarchy, resignation of the government and an end to repression and corruption. However, the Haq party wants to set up a republic.
Shia activist Abdel Wahhab Hussein and Ibrahim Sharif, who leads a secularist Sunni party, were also detained.
"Two of the thugs climbed over the fence to get in our yard, one went over and pointed a gun in Ibrahim's face and the other went to our garage to let everyone else in," Farida Ismail, Mr Sharif's wife, told Reuters news agency.
"They were going around, wrecking things in the house."
Hospital takeover Bahrain - which has a population of 800,000 and is home to the US Navy's Fifth Fleet - is the first Gulf country to be thrown into turmoil by the wave of unrest sweeping the Arab world. Protests began there last month.
On Tuesday King Hamad Bin Isa al-Khalifa declared a three-month state of emergency and Saudi troops were called in to keep order.
The following day troops moved in at dawn, backed by tanks and helicopters, to clear the central Pearl Square, where protesters had been camped for several weeks. Some Shia villages have been sealed off.
Troops also surrounded the main Salmaniya hospital, preventing the arrival and departure of doctors and injured protesters.
There are reports that they remain trapped, and that six medical staff were beaten as they tried to leave. Other doctors have said they fear they may have been arrested.
The government said it had taken over the hospital because it had become a stronghold of the opposition.
There was a call for further protests in mid-afternoon. But shortly afterwards a military officer announced a 1600 to 0400 (1300 to 0100 GMT) curfew live on TV, and there were no reports of further demonstrations.