A three-month state of emergency was declared Tuesday in the Kingdom of Bahrain amid intense pro-democracy uprisings that have threatened to topple the U.S.-backed regime.
King Hamad bin Isa Al Khalifa authorized Bahrain's armed forces "to take necessary steps to restore national security."
The martial law-style order came as Gulf allies sent more than 1,000 Saudi troops into the region in an attempt to quell the anti-government protests.
See a special AP Interactive on the uprising in the Middle East and North Africa.
Gulf leaders are concerned that the Bahraini unrest could spill over and lead to similar protests in their own countries.
Opposition groups condemned the military move, calling it an occupation and saying it pushes Bahrain toward "undeclared war."
Clashes between the two groups intensified Tuesday. A doctor told the AP that hundreds of demonstrators were injured by shotgun blasts and clubs. One demonstrator was also killed.
A Saudi official claimed one of his country's soldiers was shot and killed by a protestor.
As the security situation continues to deteriorate, the U.S. has warned Americans to avoid travel to the nation because of the "potential for ongoing political and civil unrest."
"While demonstrations have not been directed toward Westerners, U.S. citizens are urged to remain alert to local security developments and to be vigilant regarding their personal security," the State Department said in its travel warning, upgraded from an earlier travel alert.
The U.S. also urged Americans "to avoid all demonstrations, as even peaceful ones can quickly become unruly and a foreigner could become a target of harassment or worse."