A Shi'ite dissident flew home to Bahrain from exile to join an opposition movement demanding the kingdom's Sunni ruling family introduce major reforms.
Hassan Mashaima has long opposed the ruling Sunni dynasty and was sentenced in absentia last year on charges of trying to overthrow the government.
"We want a real constitution," he told reporters at the airport. "They've promised us one before and then did whatever they wanted to.
"I'm here to see what are the demands of the people at the square and sit with them and talk to them," he said, referring to anti-government protesters camped in Manama's Pearl Square.
Thousands of anti-government protesters marched from Pearl Square to a former office of prime minister Sheikh Khalifa bin Salman al-Khalifa on Saturday, in a new tactic to press demands for the removal of a man who has held his post for 40 years.
Sheikh Khalifa, the king's uncle, is a symbol of the ruling family's political power and wealth.
The protesters halted at a compound that also houses the Foreign Ministry.
Mr Mushaimaa, a London-based leader of the Shiite Haq movement, had been on trial in his absence over an alleged coup plot.
The other defendants in the case were freed in Bahrain this week and the state's foreign minister said Mr Mushaimaa had received a royal pardon and could return home unhindered.
Letting Mr Mushaimaa back was the latest in a series of concessions by the ruling al-Khalifa family aimed at placating Bahrain's majority Shiites, who have been at the forefront of nearly two weeks of protests demanding more say in government.
Tens of thousands had thronged the streets of Manama on Friday, declared a day of mourning by the government, in one of the biggest demonstrations since a "Day of Rage" on 14 February.
Security forces did not interfere. Last week seven people were killed and hundreds wounded in unrest before Bahraini rulers, under pressure from their Western allies, pledged to allow peaceful protests and offered dialogue with opponents.