Bahrain has declared a three-month state of emergency giving its military chief wide powers to battle the Shiite-led protest movement that has threatened the Sunni monarchy.
The martial law-style order from king Hamad bin Isa al-Khalifa was read out on state TV the day after more than 1,000 Saudi-led troops arrived to help prop up the regime in the first major cross-border military action to challenge one of the revolts sweeping across the Arab world.
Clashes broke out across the tiny island nation, with a doctor reporting that hundreds of protesters were injured by shotgun blasts and clubs and that one died from a bullet to the head. One of the Saudi soldiers was also shot and killed by a protester.
The foreign troops are from the six-nation Gulf Co-operation Council's Peninsula Shield Force. The bloc is made up of Saudi Arabia, Oman, Kuwait, Bahrain, Qatar and the United Arab Emirates.
Further underlining the regional implications of the unrest in Bahrain, Shiite power Iran denounced the intervention of foreign troops as "unacceptable" and predicted it would complicate the kingdom's political crisis.
Iran holds no deep political ties to Bahrain's Shiite groups, but some Iranian hard-liners have hailed their efforts over the years for greater rights for their community, which represents a majority of the nation's population. In the month of protests, the Shiite-led opposition is also pressing for political freedoms.
The United States bases its Navy's 5th Fleet in the country in part to try to counter Iran's military reach.
Other Gulf leaders have urged Bahrain's king not to give ground, fearing that gains by Bahrain's Shiite Muslims could offer a window for Iran to expand its influence on the Arab side of the Gulf. There are also worries that concessions could embolden more protests against their own regimes, which have already confronted pro-reform cries in Oman, Kuwait and Saudi Arabia.
Meanwhile, the UK government urged all UK nationals to leave the country as soon as possible and said it would help evacuate Britons if necessary.
The Foreign Office, which estimates there to be several thousand Britons in Bahrain, updated its advice telling travellers: "The overall level of the advice has changed; we advise against all travel to Bahrain; we recommend those who do not have a pressing reason to remain should leave by commercial means as soon as it is safe to do so."