Tens of thousands of Bahraini anti-government protesters displaying national flags, march to the Pearl roundabout in Manama, Bahrain, Friday, 25 February 2011. (AP)
"The time has come for true unity and our priority today is for the opposition to sit down with the protesters at Pearl Square and clearly set our demands," Opposition leader Hassan Mashaima, who had been in Britain, told reporters at his home.
The Shiite leader was among 25 men in Bahrain who had been charged in October with forming an illegal organisation, engaging in and financing terrorism and spreading false and misleading information.
Mashaima was in Britain for medical treatment when the charges were pressed last year. He had remained there until the group was granted royal pardon this week.
Lebanese authorities had arrested Mashaima on Tuesday because of an outstanding arrest warrant against him by Interpol before releasing him Friday after confirmation of his pardon.
King Hamad's pardon came amid daily protests by Bahrain's majority Shiite community, which complains of discrimination and is pushing for the Sunni al-Khalifa dynasty to be replaced by an elected government.
Manama's Pearl Square has been transformed into a makeshift camp where protesters have kept daily kept vigil in hundreds of tents.
Protesters on Saturday massed in a Manama street outside the walled compound where the Foreign Ministry is located, chanting "Down, down Hamad!"
"Leave Hamad, leave Hamad," shouted thousands of flag-waving demonstrators as they poured out of Pearl Square, epicentre of the protests which began on February 14.
"The people want to topple the regime," they roared, forming a procession towards a major highway, blocking traffic.
Official opposition groups have stopped short of demanding outright regime change, instead calling for major reforms, including an elected prime minister and the creation of a "real" constitutional monarchy.
Seven people were killed last week in a police raid on a protests against the al-Khalifa dynasty, which has ruled for some 200 years in the majority Shiite kingdom.
Bahraini opposition MPs said on Saturday they were still awaiting details on a proposed dialogue with the government before agreeing to the talks.
"Until now the government did not give any (specific) initiative for political reform," said Mattar Mattar, one of 18 Shiite MPs who withdrew from parliament in protest at the killings last week.
"Our target was declared very clearly: we want an elected government, and we want the people to write their constitution themselves through an elected council," Mattar, a member of the Al-Wefaq opposition bloc, told AFP.
"One of the most important preconditions... is that the government needs to resign first," added Al-Wefaq MP Ali al-Aswad.
King Hamad has charged Crown Prince Salman with opening dialogue with all factions in the Gulf kingdom but the talks have yet to be launched.