Riot police disperse Bahraini anti-government protesters with teargas and stun grenades in the main market in Manama.

Manama - Riot police firing teargas and stun grenades routed protesters in Bahrain's capital on Tuesday as the government came under mounting international pressure to release jailed leaders of last year's uprising.

An appeals court decision on Monday to grant a retrial to 21 opposition figures was not enough to defuse resurgent unrest among the Gulf Arab state's majority Shi'a Muslims, and street rallies resumed on Tuesday.

A heavy riot police presence cut short a demonstration in the market area of Manama as tear gas and stun grenades were unleashed at several dozen who chanted anti-government slogans.

Activists also reported small rallies on the occasion of World Labour Day in a number of Shi'a districts. Several thousand protesters and suspected supporters were sacked or suspended from work last year during a crackdown on the uprising and some say they have not got their jobs back.

Shi'as, whose unrest is seen by the Sunni Muslim ruling elite as a subversive bid to put US-aligned Bahrain under the sway of Shi'a Iran, complain of discrimination and marginalisation in political and economic life.

The government says many Shi'as hold state posts and help run the economy and that police and judicial reforms have begun. But there has been no progress on the main opposition demand for a parliament with full powers to legislate and form governments.

The cassation court, the highest judicial body in the Gulf Arab state, on Monday shifted the case of 21 men who were convicted in a military court to a civilian court and freed one, lesser-known prisoner. Seven of the 21 are abroad or in hiding.

But the court ruled the men would remain in jail, including Abdulhadi al-Khawaja, who has been on a hunger strike for three months.

A BBC team was given access to him on Tuesday and a published photograph showed him sitting up in bed, looking thin but alert. He was drinking fluids.

“More than a year after they were arrested, the Bahraini authorities have produced no evidence that the jailed leaders were doing anything but exercising their basic human rights,” New York-based Human Rights Watch said in a statement.

“The Court of Cassation made no reference, however, to the fact that the defendants had merely been exercising their basic human rights.”

In response, Sheikh Abdulaziz bin Mubarak Al Khalifa, adviser to the government's Information Affairs Authority, said: “We have full faith in the independent judiciary system in Bahrain and will wait to see these appeals take place. We are confident the outcome will be just.”

He said the defendants faced serious criminal charges that went beyond the exercise of basic human rights.

Rupert Colville, spokesperson for the UN human rights agency, told a news briefing in Geneva on Tuesday that it had urged Bahraini authorities to bring about the release of Khawaja.

“There is no reason for him to be held incommunicado and he should be given immediate access to his family, to the Danish ambassador... and to a doctor and a lawyer of his own choosing.”

Bahrain, where the US Fifth Fleet is based as a bulwark against Iran across the Gulf, remains in turmoil over a year after Shi'a-led protests first erupted, inspired by uprisings against autocratic regimes in Egypt and Tunisia.

Opposition parties stage big rallies every week and clashes between riot police and youth protesters break out nightly in Shi'a neighbourhoods around the island country, whose government is dominated by the Sunni Muslim al-Khalifa family.

The unrest has cracked the stability of Bahrain and spurred Saudi calls for a union of oil-exporting Gulf Arab monarchies to help counter Iranian influence and neutralise protest movements.

The Manama government brands the opposition as Shi'a Islamist extremists in the pocket of Shi'a clergy-ruled Iran. The opposition denies this, saying such accusations are a pretext to cling to unfair privileges.

Britain's Foreign Office said the new trial should be completed quickly and expressed concern for Khawaja's health, calling for an “urgent and compassionate solution”.

“We now urge the courts to move this forward urgently, with due process and transparency. We call for all other upheld convictions by the military courts to be reviewed without further delay,” the Foreign Office said in a statement.

The government also intends to put on trial 20 medics on charges of incitement to overthrow the government and trying to occupy a hospital. Rights groups say the 20 have been victimised for treating protesters wounded by security forces last year.

Opposition leaders said the protest campaign would continue until all prisoners were released and political and human rights reforms enacted.

Independent parliamentarian Osama Mohanna called for the al Khalifa prime minister, who has been in power for 41 years, to resign last week. A gym he owns was shot at by unknown assailants a few days later, the interior ministry said. - Reuters