MANAMA, Bahrain — A Saudi-led military force crossed into Bahrain on Monday to prop up the monarchy against widening demonstrations, launching the first cross-border military operation to quell unrest since the Arab world’s rebellions began in December.

Opposition groups immediately denounced the intervention as an occupation that pushed the tiny island kingdom dangerously close to a state of “undeclared war.”

Bahrain’s majority Shiite Muslims see an opportunity to rid themselves of two centuries of rule by a Sunni monarchy. But Gulf Sunni leaders worry that any cracks in Bahrain’s ruling system could threaten their own foundations. Protests are already flaring in Oman, Kuwait and even tightly ruled Saudi Arabia.

Gulf leaders are also concerned that political gains by Bahrain’s Shiites might give Shiite Iran a stepping stone to its arch-rival Saudi Arabia, connected to Bahrain by a wide causeway.

Instead, the Saudis and other members of the Gulf Cooperation Council sent forces the other way, deploying about 1,000 troops by land and air and cementing the entire six-nation alliance to the fate of Bahrain’s rulers, who are key U.S. allies as hosts of the American Navy’s 5th Fleet.

Shortly after word of the foreign military reinforcements began spreading through Bahrain, protesters swarmed into Manama’s Pearl Square, the symbolic center of the monthlong revolt. “No to occupation!” demonstrators cried in the square.