Riot police firing tear gas and stun grenades routed protesters in Bahrain's capital 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 Shia 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 to mark World Labour Day.

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.

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.

Khawaja's wife Khadija al-Musawi said that Monday's court decision does not change her husband's demand for an immediate release - which has become a centerpiece of anti-government protests in recent weeks.

'His condition to stop the hunger strike is to be free,'' said al-Musawi. 'If not, then the option is to die, and his death will be his freedom.'

Government promises

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, spokesman 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 Shia-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 Shia neighbourhoods around the island country, whose government is dominated by the Sunni 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.

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