Clashes Friday between Bahraini security forces and protesters left five policemen wounded and dozens of demonstrators suffering from tear-gas inhalation, the authorities and activists said.
"Five policemen were injured in clashes today (Friday), with one of them sustaining second degree burns from a homemade incendiary device," the ministry of interior said in a post on social networking website Twitter.
Witnesses said that hundreds of anti-regime protesters marched in small groups from the Shiite village of Sanabis towards Pearl Square, symbol of the protests earlier this year.
"Down Hamad," they chanted, referring to Bahrain's King Hamad bin Issa al-Khalifa, whose Sunni dynasty has ruled the Shiite majority kingdom for decades.
Security forces used "shotguns, rubber bullets and tear gas to disperse the crowds," said Mohammad Mascati, head of the Bahrain Youth Society for Human Rights.
"Dozens suffered from breathing difficulties due to tear-gas inhalation and were taken to homes for treatment out of fear from being arrested at hospitals," he told AFP.
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The ministry of interior said only tear gas was used.
"Security forces have not used shotguns, rubber bullets or birdshot in any clashes today. Only tear gas has been used," it said.
Security forces had deployed heavily in central Manama and surrounded Pearl Square with barbed wire barricades to prevent the protesters -- including some 50 women -- from approaching, witnesses said.
Demonstrators camped out in Pearl Square between mid-February and mid-March until security forces, boosted by a Saudi-led Gulf regiment, drove them out in a deadly crackdown that demolished a monument there.
The demonstrators on Friday were responding to online calls by activists to protest Saturday's by-elections, which were boycotted by the main Shiite opposition bloc Al-Wefaq.
The elections are for 18 seats left vacant in the 40-member parliament after MPs from Al-Wefaq quit in February to protest the crackdown on peaceful demonstrators.
Authorities say 24 people were killed in the unrest earlier this year, including four policemen. The opposition puts the death toll at 30.