Tens of thousands of people have joined an anti-government march just outside the Bahraini capital, Manama.
Video posted online showed protesters with Bahraini flags in a line stretching back hundreds of metres.
The security forces fired tear gas at a small group of protesters, but the rally was mostly peaceful.
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.
Next month, the Bahraini Grand Prix is due to be held in what the authorities are trying to promote as a return to normality in the country.
Protesters carried banners denouncing the government and calling for the release of political detainees. They chanted «Down, down Hamad», referring to King Hamad bin Isa Al Khalifa.
Sheikh Isa Qassim, a leading Shia cleric, made a speech at the start of the march in the village of Diraz before joining in.
Reports say the march stretched for more than 1.5km (a mile). 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.
Police used tear gas to drive back a small group of demonstrators who attempted to approach Pearl Square, which was the heart of the 2011 protests before it was stormed by security forces in March 2011.
Bahrain’s majority Shia community has long complained of deep discrimination. They make up about 70% of the country’s 525,000 citizens.
Protesters, most of them Shia, are seeking the end of the rule of the Al Khalifas, a Sunni dynasty that controls politics and all the main posts in Bahrain.
The government has offered some political concessions, but insists on controlling all the main appointments and ministries.
Bahrain is home to the US Navy’s Fifth Fleet and it is a significant oil and gas producer.
At least 50 people, including five police officers, have been killed, and hundreds arrested since the unrest began in February of last year.
The government commissioned a report into the political unrest by the Egyptian human rights expert, Cherif Bassiouni. He concluded that security personnel had used excessive force and that there had been systematic torture of detainees.
The government has claimed that Iran is behind the unrest in Bahrain, but has offered no proof of this. The claim was rejected by the Bassiouni report.