Saudi Arabian troops have crossed into Bahrain after the tiny Gulf kingdom's ruling family asked for help from neighbouring Sunni Arab states to quell a two-month uprising which threatens their 200-year-old dynasty.

The Saudi capital, Riyadh, said that it had responded to a "security threat" by deploying its troops on the streets of its neighbour. They are to protect strategic sites such as bridges and government buildings. Bahrain's rulers said the Saudi forces crossed the 16-mile causeway from Saudi Arabia to the island, together a contingent of troops from the Gulf Co-operation Council. Saudi authorities did not give details of the force; some reports estimate it to be 1,000.

Bahrain's Shia majority has laid siege to the centre of the capital, Manama, since mid-February and has, in recent days, marched on government buildings and palaces.

As news of the intervention spread through Manama, the landmark Pearl roundabout in the centre of the capital, which has become a focal point for the protests, and a nearby overpass played host to tens of thousands of demonstrators, many of them prepared for fresh confrontations with troops.

More worrying for many observers than the spectre of new clashes is an escalation in the standoff between the Gulf states and Iran, which has strongly backed Bahrain's Shia majority and has long been at odds with its Gulf neighbours, especially Saudi Arabia.

Iran's foreign minister, Ali Akbar, reacted immediately to the deployment, urging Bahrain's leaders to use restraint and wisdom in defusing the steadily deteriorating security situation. Meanwhile, Bahrain's main opposition bloc, the al-Wefaq party, described the Saudi-led intervention as a "declaration of war'. And an opposition alliance urged the United Nations to intervene.

"We consider the entry of any soldier or military machinery into the Kingdom of Bahrain's air, sea or land territories a blatant occupation," al-Wefaq said.

"This real threat about the entry of Saudi and other Gulf forces into Bahrain to confront the defenceless Bahraini people puts the Bahraini people in real danger, and threatens them with an undeclared war by armed troops."

The White House said that it did not consider the entry of Saudi Arabian security forces into Bahrain to be an invasion, an implicit reference to the 1990 entry by Iraqi forces into the small state of Kuwait which triggered two wars.

""This is not an invasion of a country," White House spokesman Jay Carney said. "We urge the government of Bahrain, as we have repeatedly, as well as other Gulf Co-operation Council countries [Oman, Qatar, Kuwait, Saudi Arabia and the United Arab Emirates], to exercise restraint," Carney added.

Diplomatic relations between Washington and Bahrain's rulers have been sorely tested in the past month. US officials had initially backed the demonstrators' demands for widespread political reforms and the instalment of a constitutional monarchy, which stripped power from the regime.

Their support was partly based on a shift in US Middle East policy to favouring democracy over stability, rather than the other way around - the stated US policy for more than 60 years.

However, frenetic shuttle diplomacy between Manama and Washington, as well as urgent appeals from the Gulf states, including Saudi Arabia, has since led the US to retreat to the sidelines in the dispute.

Bahraini officials earlier this month told their US counterparts that the protests could gain unstoppable momentum if they continued to be sanctioned by the US, a development that would imperil US interests. The US has long viewed Bahrain as an important strategic ally, and bases its Fifth Fleet near Manama, viewing the kingdom as a buffer amid tensions between the US and Iran.

Bahrain's crown prince, Sheikh Salman al-Khalifa, late last month announced a dialogue with opposition parties aimed at giving the disenchanted Shia majority a greater role in the affairs of state. The talks, however, appear to have stalled, with opposition groups at odds over key demands.

Some opposition members are insisting on the overthrowing of the monarchy, while al-Wefaq has limited its calls to making the Sunni ruling family's clan-like regime more accountable in law. Shias account for 70% of Bahrain's population, but are not allowed to serve in large parts of the country's establishment, such as the military and the police. The regime has long-believed that its Shia population is susceptible to Iranian influence.

Bahrain's security forces are almost exclusively Sunni, many invited from neighbouring states and given Bahraini citizenship. A common refrain among Shia groups is that the security forces safeguard the regime, but not the people.

As central Manama once again seethed, troops and riot police were nowhere to be seen. Clashes in February left seven demonstrators dead. Dozens more were wounded, mainly by teargas and rubber bullets, when troops ousted demonstrators from Pearl roundabout on 16 February. The crown prince ordered that troops surrender the roundabout three days later as part of the regime's calls for calm.