Protesters began marching along a highway in response to a call from leading Shi'ite cleric Sheikh Isa Qassim, who urged people to renew their calls for greater democracy.
A live blog showed images of the protesters carrying banners denouncing "dictatorship" and demanding the release of detainees, Reuters reports.
"We are here for the sake of our just demands that we cannot make concessions over and we stick with them because we have sacrificed for them," Sheikh Isa Qassim said before the march during his weekly sermon in the Shi'ite village of Diraz.
He had promised to personally lead the march, his most high-profile action in more than a year of unrest.
Majority Shi'ites were in the forefront of last year's protest movement in Bahrain, which erupted about 14 February after uprisings in Egypt and Tunisia.
But the Sunni Muslim ruling Al Khalifa family crushed the protests one month later, imposing a period of martial law and bringing in Saudi and United Arab Emirates troops to restore order.
Tension has risen in recent weeks around the anniversary of the uprising, with security forces maintaining a tight grip on the traffic intersection that protesters originally occupied.
Pro-government Sunni groups have organised counter rallies, warning the authorities not to enter into a dialogue on reforms that could give the elected parliament legislative clout and the power to form governments.
Those groups look to Sunni power Saudi Arabia as a key ally and demonise the opposition as loyal to Shi'ite Iran - a charge the opposition parties deny.
King Hamad appeared to dismiss the opposition in February this year, saying they were disunited.
