Television footage showed convoys of unmarked, desert-brown armoured vehicles crossing from Saudi's Eastern Province into Bahrain, the home of the US Fifth Fleet.

The Saudi government said it had responded to a call for help from its neighbour as Saudi-led forces from the Gulf countries' joint Peninsula Shield Force crossed the causeway separating the two countries.

"The council of ministers has confirmed that it has answered a request by Bahrain for support," the Saudi government said in a statement carried by the SPA state news agency.

Under an agreement of the six-country Gulf Cooperation Council (GCC), "any harm done to the security of a member state is considered a harm done to the security of all members," it said.

The United Arab Emirates also said it had sent some 500 police to help "defuse tension," according to Foreign Minister Sheikh Abdullah bin Zayed al-Nahayan.

It was not clear if the other GCC members Kuwait, Oman and Qatar were also participating.

The Shiite-led opposition alliance said any foreign force would be treated as an invading army.

"We consider the arrival of any soldier, or military vehicle, into Bahraini territory... an overt occupation of the kingdom of Bahrain and a conspiracy against the unarmed people of Bahrain," said an opposition statement.

But authorites called on the population to "cooperate fully and to welcome" the troops.

Helicopters buzzed overhead as thousands of protesters blocked access roads to the Financial Harbour business complex, a day after more than 200 people were injured there in clashes between riot police and demonstrators.

Manama's business district appeared a ghost town as the mainly Shiite protesters pressed their calls for democratic change from the Sunni Muslim monarchy.

Police appeared to have deserted the area, while shopping malls and office towers were closed.

Protesters also maintained their sit-in at nearby Pearl Square, where activists expected a showdown with the security forces. Thousands gathered at the square on Monday evening.

"We are not intimidated," said a woman, who asked not to be named.

Most workers seemed to be following a trade union call for a general strike to protest against violence by the security forces.

As the crisis developed, White House spokesman Jay Carney on Monday called on the Gulf forces to respect the rights of the people of Bahrain.

"We urge the government of Bahrain... as well as other GCC countries, to exercise restraint and not to meet the non-violent protests of people legitimately expressing their concerns... with any kind of physical violence."

US Secretary of State Hillary Clinton expressed "deep concern about the dangerous situation," in comments to Emirati Foreign Minister Sheikh Abdullah bin Zayed al-Nahyan in, a US official said.

But Carney declined to call for the forces to be pulled out.

"This is not an invasion of a country," he said.

Sunday was the worst day of violence in the kingdom since seven people were killed at the start of anti-regime unrest a month ago.

Shiite-majority Bahrain has transformed itself into a regional financial centre as it seeks to reduce dependence on diminishing oil revenues.

But to many of the country's disenfranchised Shiites the banking district is a symbol of corruption, wealth and privilege.

The Saudi intervention came two days after US Defence Secretary Robert Gates visited Manama and urged King Hamad to undertake rapid and significant democratic reform, not just "baby steps."

Gates spoke of Washington's fears that the longer the instability dragged on the more likely Iran, a Shiite theocracy, would to try to meddle in Bahrain's affairs.

In a major concession to the opposition demands, Crown Prince Salman said Sunday he supported setting up a parliament with full powers and pledged to tackle corruption and sectarian tensions.

But "legitimate demands should not be carried out at the price of security and stability," he warned.

The opposition however has refused to negotiate until the government resigns, a condition the country's rulers have rejected.

Foreign Minister Khalid Al-Khalifa said the opposition demonstrations amounted to "wanton, gangster-style takeover of people's lives," he said.

While the mainstream opposition says it is not trying to overthrow the royal family, more radical Shiite elements have said they want a republic.

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