MANAMA - Tens of thousands of anti-government protesters flooded a major highway in Bahrain on Friday in one of the largest opposition rallies in months against the Gulf nation's rulers. Security forces fired tear gas at smaller groups attempting to reach a heavily guarded square that was once the hub of the uprising.

The march was called as a show of resolve by a Shiite-led rebellion against Bahrain's Sunni monarchy more than a year after the Arab Spring-inspired protests began. The main procession was mostly peaceful, but breakaway groups were driven back by tear gas as they headed toward Pearl Square, which was the centre of the uprising for weeks last year until it was stormed by security forces.

The demonstration is also a reply to Bahrain's Sunni leadership, which has portrayed the uprising as losing steam ahead of next month's lucrative Formula One Grand Prix car race. It was cancelled last year because of violence on the tiny island.

The march stretched for miles. Some opposition leaders estimated the crowd at nearly 100,000, which would make it one of the largest protest gatherings since the street rallies erupted in February 2011.

Bahrain's majority Shiites seek to end the Sunni dynasty's control of all main posts and policies in the strategic Gulf nation, which is home to the U.S. Navy's 5th Fleet. Shiites make up about 70 per cent of Bahrain's 525,000 citizens, but they complain of widespread discrimination and say they are blocked from high-level political and security positions.

Bahrain's rulers have offered some concessions, including granting more decision-making powers to parliament, but reject demands that include giving up the right to appoint holders of top positions, such as prime minister.

At least 45 people have been killed in the unrest, and hundreds have been arrested.

"Down, down Hamad," protesters chanted in a reference to Bahrain's King Hamad bin Isa Al Khalifa. Others carried signs in English and French denouncing the monarchy in an appeal to international media and websites.

Organizers said the march would end at a site called Freedom Square outside Bahrain's capital, Manama. Police reinforcements were sent to keep protesters from any attempts to shift toward Pearl Square, which is now ringed by razor wire and under 24-hour watch.

The unrest in Bahrain has put the U.S. in a difficult position. Washington has called for dialogue to try to ease the tensions, but it fears that pressing too hard on Bahrain's leaders might jeopardize its important military relationships. The 5th Fleet is one of the Pentagon's main Gulf forces on Iran's doorstep.

Bahrain and its Gulf Arab allies have accused Shiite Iran of maintaining links to the Bahrain opposition groups, but no direct evidence has been produced.

In Baghdad, about 2,000 Iraqi Shiites called for Bahrain's king to be banned from the Arab League summit set for the Iraqi capital later this month.

Iraq's support for Bahrain's Shiites has angered Sunni Gulf states. That was one of the reasons the League cancelled plans to hold its summit in Baghdad last year.

The protesters, followers of hard-line cleric Muqtada al-Sadr, also complained that Syrian President Bashar Assad was not invited. His Alawite religion is an offshoot of Shiite Islam.

Syria has been suspended from the Arab League because of Assad's brutal crackdown on protesters.