Manama: Bahraini protesters camped out in Manama's Pearl Square on Sunday as police held back amid growing pressure on the Sunni ruling family to open meaningful talks with the Shiite-led opposition.

"The night passed off without any problems," said Tahar, a student who had stayed up all night with dozens of other youngsters to guard the central square, which has been the focal point of the demonstrations that have rocked the small but strategic Gulf kingdom since February 14.

"We are frightened that the security forces will launch another surprise attack like they did on Thursday," he said, referring to a nocturnal police raid to clear the square which left four people dead.

Protesters flocked back to the square on Saturday after the army, which had deployed on the streets of the capital following Thursday's raid, was ordered to return to base.

Riot police fired tear gas in an unsuccessful attempt to disperse the demonstrators but then withdrew as Crown Prince Salman bin Hamad al-Khalifa, the deputy commander of armed forces, ordered police and troops alike to hold back.

The heir to the throne has been tasked by his father King Hamad with launching a sweeping dialogue with the opposition.

But emboldened by a wave of uprisings in the Arab world that has swept the strongmen of both Tunisia and Egypt from power, the opposition has raised its stakes, demanding a 'real consitutional monarchy' and the resignation of the government.

Prime Sheikh Khalifa bin Salman, an uncle of the king, has held office ever since independence from Britain in 1971 and is widely despised by the opposition.

Bahrain is the base of the US Fifth Fleet and National Security Adviser Tom Donilon spoke with the crown prince on Saturday urging him to respect human rights and launch 'meaningful' reform, the White House said.

"Mr. Donilon reiterated the president's condemnation of violence used against peaceful protesters, and expressed support for the steps that the crown prince has ordered taken to show restraint and initiate dialogue," a statement said.

"As a long-standing partner of Bahrain, the United States believes that the stability of Bahrain depends upon respect for the universal rights of the people of Bahrain, and a process of meaningful reform that is responsive to the aspirations of all Bahrainis," it added.

Regional heavyweight Saudi Arabia, meanwhile, voiced its 'absolute rejection' of any foreign meddling in Bahraini affairs, pledging to stand by its tiny neighbour.

The Bahraini crown prince acknowledged the need for reform but called for calm before the launch of dialogue.

"There are clear messages from the Bahraini people... about the need for reforms," he said in a television interview on Saturday.

A large banner erected by the protesters in Pearl Square insisted: "We do not accept dialogue with any of the murderers."

Another read "Khalifa, Go!" in reference to the veteran prime minister.

The main Shiite opposition group, the Islamic National Accord Association, which is the largest bloc in parliament, has been boycotting the legislature in protest at the security forces' deadly crackdown last week.

Bahrain's main trade union called an indefinite strike from Sunday to demand the right to demonstrate peacefully.

EU foreign affairs chief Catherine Ashton said it was vital that the promised dialogue 'should begin without delay.'

In a telephone call to the crown prince on Saturday, British Foreign Secretary William Hague said he had expressed 'the UK's deep concern about the situation and strong disapproval of the use of live ammunition against protesters.'