Security forces in Bahrain fired tear gas and stun grenades at protesters trying to occupy a landmark square in the nation's capital on Monday ahead of the one-year anniversary of the Gulf kingdom's Shiite-led uprising.

Thousands of opposition supporters marched through Manama's streets in the largest attempt in months to retake Pearl Square, the central roundabout that served as the epicenter of weeks of protests last year by Bahrain's Shiite majority against the ruling Sunni dynasty.

Thousands of riot police and other security forces have staked out positions around the square and across the Gulf island nation to prevent the opposition from staging a mass rally in or near the plaza to mark Tuesday's one-year anniversary of the revolt.

Opposition supporters were undeterred by the authorities' warnings of zero tolerance for anti-government activities around the strategic island that is the home of the U.S. Navy's 5th Fleet.

"We will not back down," said Nader Abdulimam, who had taken refuge in a house just outside of Manama with other protesters overcome by tear gas. "This has gone on for one year and it will go for another year or more."

Some protesters hurled firebombs and rocks after the security forces fired tear gas. In an area about six miles (10 kilometers) west of central Manama, some demonstrators stood atop Bahrain's ancient burial mounds _ some more than 5,000 years old _ waving flags featuring the image of Pearl Square's six-pronged monument.

More than 50 police vehicles filled a site that protesters have dubbed "Freedom Square," which hosted several government-sanctioned opposition gatherings last week.

After the government imposed martial law last March in response to the demonstrations, security forces stormed the protesters' encampment at the landmark square in a bid to crush the uprising. The authorities then razed the towering white monument that stood in the center of the plaza.

The now heavily guarded square holds great symbolic value for Bahrain's opposition movement, and protesters have repeatedly tried to reoccupy it. But authorities have effectively locked off the capital to demonstrations since March.

Emergency rule was lifted in June, but street battles between security forces and protesters still flare up almost every day in the predominantly Shiite villages around the capital.

At least 40 people have been killed during months of unprecedented political unrest in Bahrain, the Gulf country hardest hit by upheaval during last year's Arab Spring protests. Neighboring Saudi Arabia and other Sunni-ruled Gulf states dispatched troops to Bahrain in March to help crush the protests.

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.

Bahrain's Sunni rulers have promised reforms, although they refused to make the far-reaching changes the protesters and the main Shiite group, Al Wefaq, have demanded. These include ending the monarchy's ability to select the government and set key state policies.

U.S. State Department spokeswoman Victoria Nuland expressed concern over the violence Monday and said Washington wants to see demonstrators stage peaceful protests and for security forces to "exercise restraint and operate within the rule of law and international judicial standards."

In London, Amnesty International noted that Bahrain's leaders have taken some steps toward easing tensions in the past year, including reinstating hundreds of workers dismissed for protesting and ending military-run tribunals. The group urged more measures to show that the steps were "more than a public relations exercise."

Amnesty called for releasing prisoners held on protest-related offenses and moving ahead with investigations into alleged abuses by security forces and others.

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Surk reported from Dubai, United Arab Emirates. Associated Press writers Brian Murphy in Dubai and Matthew Lee in Washington contributed to this report.