Bahraini security forces fanned out across the island nation in unprecedented numbers on Tuesday as Shiites marked the one-year anniversary of their uprising against the country's Sunni rulers.
Authorities sent troop reinforcements and armored vehicles to the predominantly Shiite villages around the capital Manama to prevent people from gathering and answering the call of the main opposition movement, Al Wefaq.
Six American members of an activist group were detained during the protests, activists and the government said. Authorities said they had violated the terms of their visa, and that they agreed to leave the country without further legal measures taken against them.
The government meanwhile threatened to take legal action against the organizers of protests on Monday that turned violent. This could herald a new crackdown on Al Wefaq, which until last year was tolerated but which has suffered sporadic prosecutions and detentions after it took the lead in last year's protests.
At least 40 people have been killed during a year of unprecedented political unrest in Bahrain. The island kingdom, the home of the U.S. 5th Fleet, was the Gulf Arab nation hardest hit by upheaval during 2011's Arab Spring protests.
The kingdom's ruling dynasty has promised reforms to end the upheaval, although it refuses to make the far-reaching changes the protesters and Al Wefaq, have demanded. These include ending the monarchy's ability to select the government, set key state policies and appoint most of the parliament members.
Police on Tuesday fired tear gas at protesters in an apparent attempt to pre-empt a repeat of the marches the night before, in which protesters made their largest effort in months to retake the city's central roundabout. Pearl Square had served as the epicenter of weeks of anti-government protests last year, and its reoccupation would be a major boost for the movement.
The government statement said many protesters on Monday departed from a pre-authorized route in Manama, turning the march into a riot after police arrived. It said Al Wefaq was responsible for the violence, because it failed to "control the crowd (and) that jeopardized the safety of the people along a busy main road."
Legal procedures will be taken against the organizers of the march, Tuesday's statement said.
Al Wefaq rejected the claim, and said that the "unfounded accusations" are part of the rulers' efforts to discredit the group.
"They have used excessive force against the people throughout all this time, but people keep coming back to the streets to insist on their demand to have a role in the decisions about their country," said Abdul Jalil Khalil, a former Al Wefaq parliamentarian.
Shiites account for about 70 percent of Bahrain's population of some 525,000 people, but say they have faced decades of discrimination and are blocked from top political and security posts.
"After years of broken promises for change, Bahrain exploded last year," Khalil said. "We are still here and we want serious solution and meaningful reform."
Sunni rulers made token concessions in June ahead of U.S.-supported reconciliation talks between the monarchy and the opposition. A so-called national dialogue began in July, but Al Wefaq delegates pulled out of the talks, saying the government was not willing to discuss political reform.
Since then, no talks between the monarchy and the opposition have taken place, Khalil said. Street battles between security forces and protesters still flare up almost every day in the predominantly Shiite villages around the capital.
U.S. State Department spokeswoman Victoria Nuland said Tuesday the Bahraini government and the opposition "have a responsibility, both of them, to work together to ensure that the right to peaceful protest is respected by all sides."
Nuland also called on the demonstrators to refrain from violence and urged Bahraini security forces to "use maximum restraint" during peaceful protests. Washington asked the government in Bahrain to work with the opposition and other groups "to establish a process leading to real, meaningful political reform" in the Gulf kingdom.
The U.S.-based Witness Bahrain meanwhile said in a statement that six American members of the activist group were arrested during a peaceful march and held at a Manama police station.
Bahrain's Information Affairs Authority said that six U.S. passport holders have been ordered deported for "applying for tourist visas under false pretenses."
The statement says the six activists arrived in Bahrain in the past week and obtained tourist visas at the airport. "They were participating in illegal demonstrations," the statement says.
It said they were taken to a police station and "agreed to leave the country without further legal procedures" against them.
Another two Americans from the group were deported from Bahrain Sunday for taking part in opposition activities after they had entered the Gulf kingdom on tourist visas. Authorities on Monday tightened entry policies into the kingdom, and now demand prior visa approval for many nations that had been allowed to obtain entry stamps upon arrival, including the U.S. and other Western countries.
Surk reported from Dubai, United Arab Emirates. Associated Press writer Matthew Lee in Washington contributed to this report.