Bahraini protesters threw petrol bombs and stones on Tuesday at riot police who were trying to disperse them with water cannons after the funeral of a Shi'ite man jailed for his role in last year's pro-democracy uprising, witnesses said.
Bahrain's Information Authority said Mohammed Ali Ahmed Mushaima had been in hospital since August, and died of complications from sickle cell disease.
But opposition activists accused the authorities of causing the 23-year-old's death by denying him proper treatment.
"Due to his medical condition, his lawyer had tried more than once ... to ensure adequate medical attention for him, but the prison authorities failed to provide such attention," said Mohammed al-Maskati, head of the Bahrain Youth Society for Human Rights.
Mushaima was jailed for seven years in March 2011 for "vandalism, rioting, assaulting a police officer and resisting arrest", one of hundreds of people who were arrested in weeks of mass protests inspired by uprisings in Tunisia and Egypt.
Despite that crackdown and two months of martial law that followed, clashes between police and protesters still occur almost daily in Bahrain, where the U.S. Fifth Fleet is based. On Friday, a 17-year-old died in a confrontation with riot police.
The protesters, mainly from the Shi'ite majority, had demanded a bigger role for elected representatives and less power for the ruling al-Khalifa family, who are Sunni Muslims. Some Shi'ite groups sought an end to the monarchy.
Shi'ites complain of discrimination in the electoral system, jobs, housing, education and government departments including the police and army. They say government assertions that it is addressing those concerns have produced no action.
A commission of international legal experts reported in November that torture had been systematically used on protesters to punish and extract hundreds of confessions. Among its many recommendations were reviewing activists' jail sentences.
Witnesses said thousands of people, mostly Shi'ites, had gathered for Mushaima's funeral on Tuesday, which passed peacefully.
They said clashes erupted afterwards when police stopped hundreds of people trying to march to the Pearl Roundabout, the centre of last year's uprising.
Separately, activists said six Bahraini medics had been re-arrested on Tuesday, a day after losing appeals against jail terms ranging from one month to five years for their role in the pro-democracy protests.
The medics were freed from detention last year after an outcry over allegations of torture.
Ali al-Ekry, former senior surgeon at the Salmaniya hospital in Manama, was sentenced in June to five years in jail. Eight other medics received prison sentences ranging from one month to three years, and nine people were acquitted.
The charges included inciting hatred and calling for the overthrow of Bahrain's rulers.
The World Medical Association, a confederation of 100 national medical associations, said the verdicts were "unacceptable" and urged Bahrain to drop the sentences.
Amnesty said it considered the jailed medics prisoners of conscience.
"Despite the government's claims that the medics committed a criminal offence, Amnesty International believes they have been jailed solely for peacefully exercising their legitimate rights to freedom of expression, association and assembly," said Ann Harrison, programme director for the Middle East and North Africa at Amnesty International.
Bahraini officials could not be reached immediately for comment on the cases.
The government has denounced the protest movement as sectarian and part of a quest by Shi'ite Iran to dominate the region. Iran denies involvement and Bahraini Shi'ites deny being steered from Tehran.