Bahrain rounded up dissidents Thursday as it came under mounting diplomatic pressure to end a bloody crackdown on Shiite-led protesters, which has alarmed its ally the United States and infuriated Iran.

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.

"They were arrested in the night," Khalil Marzouk, deputy leader of the Al-Wefaq opposition movement, told AFP.

Among those detained was Hassan Mashaima, a leader of the hardline Shiite 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 26 February after terrorism charges against him were dropped as part of an earlier peace offering from the government to the opposition.

Human rights activist and Haq member Abduljalil al-Singace, who was released in February after six months in jail, was also detained 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.

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 policemen died in hit-and-run attacks by opposition motorists.

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."

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

King Hamad declared a three-month state of emergency on Tuesday and hundreds of 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 the Gulf states' military deployment as "the wrong track."

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

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

A dusk-to-dawn curfew was slapped on Manama's business district, and movement was restricted. Manama's main hospital was sealed off by police armed with shotguns, while banks and businesses remained closed.

Rights activists have accused the security forces of preventing the injured from reaching hospitals and of beating medics trying to collect the wounded from the streets.

Bahrain's Health Minister Nizar Baharna, a Shiite, announced his resignation after police allegedly burst into a Manama hospital, and 12 Shiite judges also stepped down in protest at what they termed the "excessive use of force."

Amnesty International's regional director Malcolm Smart said reports coming out of Bahrain were evidence the authorities were "using lethal and other excessive force to crush protests, with reckless disregard for human life."

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, also appealed to Bahraini authorities to stop the violence, and Hezbollah supporters rallied in Lebanon to denounce the Saudi intervention.

The United States and Britain have urged citizens to leave the country and The Pentagon has authorised the departure of non-emergency Defense Department staff.

Britain said Thursday said it was chartering planes to help its stranded citizens flee the country.