Thousands of anti-government marchers have jammed a major highway Friday as clashes broke out for a second day between security forces and protesters marking the anniversary of their uprising in the strategic Gulf nation.

The spike in violence is likely to put pressure on Shia opposition groups as they engage in talks to ease the crisis with Bahrain's Sunni-led government, despite objections from hard-line factions who want to topple the Western-backed monarchy.

In another sign of escalating tensions, police said they found a bomb weighing two kilograms on the causeway connecting Bahrain and Saudi Arabia, which closely backs Bahrain's monarchy. Police said bomb disposal teams defused the device, which officials said contained "highly explosive material."

A 16-year-old boy and a policeman were killed on Thursday — the second anniversary of the Shia-led uprising that seeks a greater political voice in the affairs of the kingdom, which is home to the U.S. Navy's 5th Fleet.

Bahrain's Shias comprise about 70 per cent of Bahrain's more than 550,000 native-born population, but claim they face widespread discrimination and are blocked from key political and military roles. Nearly 60 people have died in the unrest. Some activist groups place the toll higher.

The protesters joined a peaceful march along a main highway linking the capital, Manama, with Shia areas to the west. Breakaway groups, meanwhile, clashed with riot police in nearby neighbourhoods.

Main Shia political groups opened talks this month with government and Sunni envoys to try to ease the crisis. Washington and other Western allies of Bahrain's rulers have applauded the effort. But some opposition factions oppose the dialogue, claiming it will not weaken the Sunni dynasty's power.

Officials said policeman Mohammed Asif was fatally wounded from a "projectile" after attacks from demonstrators. An earlier statement blamed Asif's death late Thursday on a "domestic terror act."

Meanwhile, a separate investigation is under way into the death of the teenager. Activists said he was killed on Thursday by police birdshot fire.