MANAMA (AFP) – The United Nations warned on Thursday of "shocking and illegal" abuses in Bahrain after a bloody crackdown on Shiite-led protesters which has alarmed the United States and infuriated the Shiite world.

Political dissidents were rounded up at gunpoint in midnight raids and police armed with shot-guns stood outside Manama's main hospital, amid reports the authorities were beating doctors and denying treatment to the wounded.

UN rights chief Navi Pillay said any takeover by the security forces of hospitals and medical facilities was "a blatant violation of international law."

"Governments are obliged to protect the rights to life and health of the people, but we are hearing very credible reports indicating that they are in fact obstructing access to such rights," she said.

"There are reports of arbitrary arrests, killings, beatings of protesters and of medical personnel, and of the takeover of hospitals and medical centres by various security forces.

"This is shocking and illegal conduct."

Five hardline Shiite activists and one Sunni were arrested during the night, a parliamentarian from the Shiite opposition alliance said, after a day of violence which left five dead in the Sunni-ruled kingdom.

Security forces firing tear gas and shotguns cleared out a pro-democracy sit-in at Manama's Pearl Square on Wednesday in the worst day of violence since activists took to the streets last month.

The opposition said three demonstrators were killed in the raid, while the government said two police died in hit-and-run attacks by opposition motorists.

Hundreds have been wounded in clashes in recent days, witnesses and medics said.

The protesters are demanding a constitutional monarchy, the resignation of the government and an end to repression and corruption. Others like the radical Shiite Haq group want a republic.

US President Barack Obama, whose country is a close ally of Bahrain, called King Hamad to express "deep concern," while British Prime Minister David Cameron urged the king to pursue "reform, not repression."

King Hamad declared a three-month state of emergency on Tuesday and more than 1,000 armoured troops from Saudi Arabia and the United Arab Emirates have entered the country to help restore security.

The unrest triggered jitters on oil markets where New York's main contract, light sweet crude for delivery in April, gained 43 cents to $98.41 per barrel in Asian trade on Thursday, dealers said.

The sectarian tensions have given rise to fears among Bahrain's Sunni-led Arab neighbours that Iran, the Shiite power a short boat ride away across the Gulf, is seeking to foment unrest in the kingdom.

US Secretary of State Hillary Clinton described the situation in Bahrain as "alarming" and criticised Washington's Gulf state allies for heading down "the wrong track" of military intervention.

Bahrain is the home of the US Fifth Fleet and has become a regional financial hub as its seeks to diversify its economy away from a reliance on diminishing oil reserves.

Among those detained overnight was Hassan Mashaima, a leader of the Haq group which is seeking to overthrow the Sunni monarchy that has ruled the Shiite-majority island state for 230 years.

Mashaima only returned to Manama from abroad on February 26 after terrorism charges against him were dropped as part of a peace offering from the government.

Human rights activist and Haq member Abduljalil al-Singace, who was released in February after six months in jail, was also arrested along with leftist activist Ibrahim Sharif, the opposition said.

The government has not confirmed the arrests.

"Four men arrived around two in the morning. One of them put a revolver to my husband's temple and took him away without even giving us time to call his lawyer," Sharif's wife, Farida Gulam, told AFP.

Police and troops have fanned out across Manama and Shiite villages in the surrounding countryside were sealed off amid reports of clashes.

A dusk-to-dawn curfew was slapped on Manama's business district, movement was restricted and tanks guarded key areas.

But on Thursday the army eased the curfew, saying it would now take effect four hours later at 8:00 pm and end at 4:00 am.

Health Minister Nizar Baharna, a Shiite, said he quit after police raided Salmaniya hospital in what state television reported as an operation to "cleanse" it of "saboteurs."

Twelve Shiite judges also stepped down to protest the "excessive use of force."

Iranian President Mahmoud Ahmadinejad on Wednesday condemned the intervention of Saudi-led Gulf forces to prop up the Al-Khalifa royal family as "foul and doomed," and Tehran withdrew its ambassador from Bahrain.

The spiritual guide of Iraq's majority Shiites, Grand Ayatollah Ali al-Sistani, and Lebanon's Shiite Hezbollah militia also denounce the Saudi army intervention.

The US Pentagon has authorised the departure of non-emergency Defense Department staff while Britain said it was chartering planes to help evacuate its citizens.