The force marked the first cross-border military operation to quell unrest since the Arab world's rebellions began in December.
Bahrain's main opposition groups immediately denounced the outside intervention as an "occupation" that pushed the tiny Gulf kingdom dangerously close to a state of war.
The Gulf Sunni dynasties are fearful for their own fate as the Arab push for change rumbles through the oil-rich region. They also see any gains by Bahrain's Shiites as a potential foothold for Shiite heavyweight Iran to increase influence, including with Saudi Arabia's restive Shiite minority in areas just over the causeway from the island of Bahrain.
"The Bahrain government asked us yesterday to look at ways to help them to defuse tension in Bahrain," United Arab Emirates Foreign Minister Abdullah bin Zayed Al-Nahyan said in Paris. He said they sent 500 Emirati police and the Saudis and others also sent forces "to get calm and order in Bahrain." The strife in Bahrain escalated dramatically over the weekend just as U.S. Defense Secretary Robert Gates arrived to urge its leaders key Washington allies to heed at least some of the demands for change.
A Saudi security official said the Gulf units dispatched to Bahrain come from a special force within the six-nation Gulf Cooperation Council.
He did not give details on the size or national breakdown of the force estimated in some reports at about 1,000 strong but said they were deployed by air and road and will help protect key buildings in the strategic nation, which hosts the U.S. Navy's 5th Fleet.
The official spoke on condition of anonymity because he was not authorised to brief media. The GCC members are Bahrain, Saudi Arabia, Kuwait, Qatar, Oman and the United Arab Emirates.
The Gulf Daily News, which is close to Bahrain's rulers, said the outside forces would protect sites such as electricity stations and oil facilities.
The arrival of the military force comes a day after some of the most widespread chaos in the month-long series of protests and clashes that have left seven dead and the nation deeply divided.
On Sunday, protesters blocked the main route to Bahrain's important financial district and battled pro-government mobs at the main university, which has cancelled classed indefinitely.