A three-month state of emergency was declared by Bahrain's King Hamad bin Isa Al Khalifahtoday, authorizing the head of the military to "to take necessary steps to restore national security."
It is the latest escalation in the tense and often violent month-long standoff between Shiite Muslim protesters and the security forces of the ruling Sunni family.
The declaration of martial law comes a day after a taskforce of around 1,500 troops from the Gulf Cooperation Council arrived in Bahrain to bolster the tiny island nation's forces. Most are from Saudi Arabia, crossing the short causeway that connects the two kingdoms.
The presence of foreign troops has infuriated the demonstrators who marched today from their symbolic home base of Pearl Square towards the Saudi embassy.
The last three days have seen the most violent demonstrations since they first began in mid-February. Graphic videos and photos circulated online today purporting to show at least three protesters killed.
Meanwhile, the Associated Press reported that a Saudi soldier was shot dead by a protester, and Agence France-Presse said a Bahraini policeman was killed, bringing the overall death toll to at least 12.
On Sunday, protesters barricaded the King Faisal Highway, blocking access to Bahrain's Financial Harbor, home to many banks and institutions. Bahrain's Crown Prince Salman bin Hamad bin Isa Al-Khalifa went on state television that night, calling for dialogue but saying that "the right to security and stability transcends any other consideration."
Iran is lining up behind the Shiite protesters, calling the presence of foreign troops in Bahrain "unacceptable."
"People have some legitimate demands and they are expressing them peacefully. It should not be responded to violently," an Iranian foreign ministry spokesman said.
There are concerns that Bahrain's unrest could develop into a proxy war between Iran and its Sunni Arab neighbors. Iran has been accused of backing the protesters but so far no evidence has been offered.
What began as protests by the Shiite minority for more rights and a constitutional monarchy has developed into calls for the monarchy to be abolished. Bahrain is a key ally for the US, the home of the Navy's 5th Fleet. During a recent visit, Secretary of Defense Robert Gates urged the king to undertake real reforms, not "baby steps."
"We urge our GCC partners to show restraint and respect the rights of the people of Bahrain, and to act in a way that supports dialogue instead of undermining it," White House spokesman Tommy Vietor said Monday.
The U.S. State Department is urging Americans to avoid travel to Bahrain and suggesting those there to leave. The embassy in the capital Manama has authorized the departure of non-essential personnel.