A 16-year-old protester died after what opposition activists in Bahrain said was a "brutal attack" by security forces.
The Bahrain government described the incident as a defensive response to a petrol bomb attack on police.
Opposition parties in Bahrain say more than 45 people have been killed in protests since June 2011, when the government lifted the martial law it imposed to help quash pro-democracy demonstrations by its Shia Muslim majority inspired by revolts against repressive dynasties across the Arab world.
The interior ministry says protesters have injured more than 700 police officers in clashes and that the police, who do not use live fire, are practising restraint.
The protester's death after the protest last night came as the United States, an ally of Manama, expressed concern over Bahrain's jailing of a prominent opposition activist, Nabeel Rajab, for three years.
The government identified the dead youth as 16-year-old Hussam al-Haddad, and said he had been among protesters throwing petrol bombs at police and had died after being taken to hospital.
The opposition Bahrain Centre for Human Rights said witnesses had seen the security forces firing gunshot pellets at Haddad before men in plainclothes kicked him repeatedly as he lay on the ground while police stood by. The main opposition Wefaq movement said in a statement in Arabic that Haddad had been "martyred after being brutally attacked".
The government said police were acting in self defence.
"Terrorists launched petrol bombs at close range, forcing the police to take the necessary actions to defend themselves and innocent bystanders from the potentially lethal attack," the government information affairs authority said.
"Despite warning shots by the police the attack continued; so security personnel dealt with the case according to its legal authority," it added, citing the police chief of Muharraq district where the incident took place.
Rights groups have accused the government of firing teargas canisters at close range and into confined spaces as a weapon instead of using the gas solely to disperse protests, something the government has denied.
Bahrain's Shia Muslims say they have been marginalised in the tiny island kingdom's political and economic life, but the Sunni-led government denies this.
The ruling family has rejected the main opposition demand for an elected parliament with full powers to pass laws and form governments, but has carried out reforms to increase parliamentary scrutiny over ministers and has said it will reform the police.