MANAMA – Two people were killed and at least 50 injured overnight Wednesday as Bahrain's security forces moved in to clear protesters camped out in a central Manama square, a Shiite opposition spokesman told AFP.

"Two people have been killed and at least 50 others injured, at least 10 of them seriously," Matar Matar, spokesman for the Al-Wefaq movement, the main Shiite opposition group said.

Bahraini security forces used tear gas as they moved in on the protesters, witnesses told AFP, and explosions and ambulance sirens could be heard a few hundred metres (yards) from the square, which had been sealed off.

Journalists saw the protesters leaving the square pursued by the security forces, as a helicopter flew over the area.

"They attacked the square where hundreds of people were spending the night in tents," said one witness, 37-year-old Fadel Ahmad.

"They came in over a bridge that overhangs the square," said another demonstrator, Mahmoud Faraj, who had fled the scene.

"There are several people injured," he added.

Another protester said tens of injured people had been hospitalised.

By dawn Thursday, the square was nearly empty apart from a few people being questioned by police officers.

Thousands of demonstrators had been occupying Manama's Pearl Square since Tuesday after police killed two young Shiite demonstrators during anti-government protests.

They had renamed it Tahrir (Liberation) Square, after the one in Cairo that became the focal point of the uprising that finally toppled strongman Hosni Mubarak last Friday after 18 days of nationwide protests.

On Wednesday, thousands of Bahrainis chanted for a change of regime and a "real constitutional monarchy" after the burial of the second protester.

The interior ministry had said it would allow demonstrators to stay in Pearl Square, "taking in consideration the feelings" of the people.

The White House said Wednesday it was watching unrest in Bahrain "very closely" and called on its rulers to allow peaceful anti-government protests.

"Bahrain, like all the countries in the region, needs to respect the universal rights of its citizens, their right to protest, their right to have their grievances heard," White House spokesman Jay Carney said.

"They should refrain from violence on both sides. We are obviously watching events from Bahrain and around the region very closely.

Bahrain serves as headquarters for a pillar of American military power, the US Navy's Fifth Fleet, which commands a rotating flotilla of vessels charged with safeguarding oil shipping lanes in the Gulf and countering Iran.

The US Navy announced Wednesday the aircraft carrier USS Enterprise had crossed the Suez Canal on its way to the Gulf.

The Bahrain protests started on Monday after activists posted message on Facebook calling for demonstrations to press for political reform in the wake of successful uprising in Tunisia and Egypt.