RIYADH - Saudi Arabia sent troops into Bahrain on Monday to help put down weeks of protests by the Shi'ite Muslim majority, a move opponents of the Sunni ruling family on the island called a declaration of war.

Analysts saw the troop movement as a mark of concern in Saudi Arabia that political concessions by Bahrain's monarchy could embolden the Saudi kingdom's own Shi'ite minority.

About 1,000 Saudi soldiers entered Bahrain to protect government facilities, a Saudi official source said, a day after mainly Shi'ite protesters overran police and blocked roads.

"They are part of the Gulf Cooperation Council (GCC) force that would guard the government installations," the source said, referring to the six-member bloc that coordinates military and economic policy in the world's top oil-exporting region.

Witnesses said the 25-km (16-mile) causeway between the two countries was closed and tanks were rolling across to Bahrain, a key U.S. ally and home of the US Navy's Fifth Fleet.

Analysts and diplomats say the largest contingent in any GCC force would come from Saudi Arabia, which is already worried an uprising by Bahrain's Shi'ites may inspire restive Shi'ites in its own Eastern Province, the centre of the oil industry.

Bahraini opposition groups including the largest Shi'ite party Wefaq said the move was an attack on defenceless citizens.

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

"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 reports come after Bahraini police clashed on Sunday with mostly Shi'ite demonstrators in one of the most violent confrontations since troops killed seven protesters last month.

After trying to push back demonstrators for several hours, police backed away and youths built barricades across the highway to the main financial district of the Gulf banking hub.

Those barricades were still up on Monday morning, with protesters checking cars at the entrance to the Pearl roundabout, the focal point of weeks of protests. On the other side of the same highway, police set up a roadblock preventing any cars moving from the airport towards the harbour.

Police were out in force in some areas but there was no evidence of soldiers, Bahraini or otherwise in Manama.

"We will never leave. This is our country," said Abdullah, a protester, when asked if Saudi troops would stop them. "Why should we be afraid? We are not afraid in our country."

SECTARIAN CONFLICT

Bahrain has been gripped by its worst unrest since the 1990s after protesters took to the streets last month, inspired by uprisings that toppled the leaders of Egypt and Tunisia.

Any intervention by Gulf Arab troops in Bahrain is highly sensitive on the island, where the Shi'ite Muslim majority complains of discrimination by the Sunni Muslim royal family.

Most Gulf Arab ruling families are Sunni and intervention could be seen by local Shi'ites as an assault. This, in turn, might encourage intervention by non-Arab Iran, the main Shi'ite power in the region. Accusations already abound of Iranian backing for Shi'ite activists in Bahrain -- charges they deny.

"Shi'ites in states with large Shi'ite populations, in particular Kuwait and Saudi Arabia, may intensify their own local anti-regime demonstrations," said Ghanem Nuseibeh, partner at consultancy Cornerstone Global.

"The Bahraini unrest could potentially turn into regional sectarian violence that goes beyond the borders of the particular states concerned."

In a sign that the opposition and the royals may find an 11th hour solution, the opposition groups said they had met the crown prince to discuss the mechanism for national dialogue.

Crown Prince Sheikh Salman al-Khalifa offered assurances on Sunday that dialogue would address key opposition demands including giving parliament more power and reforming government and electoral districts.

Even if talks are successful however, the opposition is increasingly split and hardline groups may keep up protests.

Wefaq is calling for a new government and a constitutional monarchy that vests the judicial, executive and legislative authority with the people. A coalition of much smaller Shi'ite parties are calling for the overthrow of the monarchy -- demands that scare Sunnis who fear this would benefit Iran.

Thousands of protesters still occupy Pearl roundabout. Seven people were killed early in the protests, when troops opened fire and cleared the roundabout. Washington has since urged restraint.

Speaking in Riyadh on Sunday, GCC Secretary-General Abdul Rahman al-Attiyah said that ensuring security and stability in any Gulf Arab country was the responsibility of the whole bloc.

In an apparent reference to Iran, however, he said the GCC rejected foreign meddling in Bahrain and that any effort to destabilise the country was a "dangerous encroachment on the security and stability" of the whole region.