About 1000 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.
Bahrain said on Monday it had asked the Gulf troops for support in line with a Gulf Cooperation Council (GCC) defence pact. The United Arab Emirates says it will also send 500 police to Bahrain.
The arrival of the Saudi forces came after mostly Shi'ite demonstrators overwhelmed Bahraini police on Sunday and blocked the highway to the main financial district in the most violent confrontations since troops killed seven protesters last month.
Witnesses saw some 150 light armoured troop carriers, ambulances, water tankers and jeeps cross into Bahrain via the 25km causeway and head towards Riffa, a Sunni area that is home to the royal family and military hospital.
Bahrain TV later showed footage it said was of advance units of the joint regional Peninsula Shield forces that had arrived in Bahrain. It later said a second wave of forces had arrived.
Bahraini opposition groups, including the largest Shi'ite party Wefaq, say the move is an attack on defenceless citizens.
Surprise to US
The White House says it had no warning of the Saudi reinforcements.
It says the United States does not consider the Saudi security forces constitute an invasion, but urged the Bahrain government to exercise restraint in upholding order.
The Pentagon says it has expressed concern to all parties about any action that could inflame sectarian tensions in Bahrain.
This is the first time any Arab government has called for outside military help during the current wave of protests sweeping the region.