More protests could jeopardize Bahrain GP
They began marching along a highway near Manama in response to a call from leading Shiite 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.
"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 in his weekly sermon in the Shiite village of Diraz.
"It is the biggest demonstration in the past year. I would say it could be over 100,000," said a Reuters photographer after protesters filled up the main Budaiya highway in the area of Diraz and Saar, west of Manama.
Security forces fired tear gas at a small group of protesters, but the rally was mostly peaceful, the BBC reported.
Activists had called for the biggest rally since the Bahraini authorities quelled a popular protest with help from Saudi troops more than a year ago.
Later, hundreds of protesters broke away from the march to walk down the main highway into Manama in an attempt to return to a traffic intersection that protesters occupied for a month during last year's uprising.
Activists said riot police blocking the road fired tear gas and the interior ministry said protesters threw stones.
The government, pressed by its Western allies to allow peaceful expression of dissent, has allowed more opposition protests in recent months.
The BBC reported some protesters chanted "Down, down Hamad," referring to King Hamad bin Isa Al Khalifa.
A statement from the royal court praised a small Friday rally of several hundred government loyalists under the name "Fateh Gathering", and the Qassim march, as signs of democratic maturity.
"The events at the Fateh Gathering as well as the gathering in the Northern Governorate are a source of pride for Bahrainis as a model of correct democratic behavior," state news agency BNA reported.