A Bahraini appeal court has ordered the retrial of two men sentenced to death for running over and killing two policemen during pro-democracy protests last year, state media reported.

However, a U.S. rights campaigner said he had been prevented from entering Bahrain to observe a separate appeal hearing this week for medical workers jailed on charges including incitement to overthrow the government during the protests.

Bahrain remains in crisis after the Sunni Muslim monarchy repressed the protests led by majority Shi'ites by force last year. Demonstrators continue to clash daily with police but in smaller protests scattered throughout the Gulf island state.

Courts are reviewing many cases after a rights commission, formed by King Hamad bin Isa Al Khalifa after international criticism of the crackdown, talked of torture and abuse during over two months of martial law.

The state news agency BNA said the court of cassation, the kingdom's highest appeals court, had overturned the death sentences for the two Bahrainis on Monday as well as a life jail sentence for one other.

The original verdicts were handed down by a military court in May but the Bahrain Independent Commission of Inquiry (BICI) criticised the military courts in its report issued in November.

The men face a retrial in a civil court, BNA said. The incident happened during an operation to retake control of a traffic roundabout in Manama which had become the centre of the protests in February and March.

Bahrain hosts the U.S. Fifth Fleet that patrols the Gulf from where a large portion of world oil supplies are shipped. The government says Shi'ite power Iran is fomenting the unrest, a charge opposition figures and activists deny.

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Opposition groups are demanding a retrial or revoking of sentences in other cases. These include that of 21 political figures, rights activists and a blogger accused of leading the protests to change the political system violently. Eight of them received life sentences.

Rick Sollom of the U.S.-based organisation Physicians for Human Rights said in message on Twitter that he had been held at Manama airport this week for hours and refused entry.

Sollom had wanted to attend an appeal by 20 medical workers sentenced to 5 to 15 years on charges including incitement to overthrow the government and attempting to occupy a hospital.

The Bahrain Government continues to undermine its stated commitment to human rights reform by holding sham trials, attacking human rights defenders and denying access to international observers, said Brian Dooley of U.S. group Human Rights First in a statement criticising Sollom's treatment.

A statement by the human rights ministry said Sollom would be welcome after February 22, the deadline the government has set itself for implementing the BICI recommendations.

The medics' trial provoked heavy international criticism, prompting prosecutors to order an appeal hearing in a civilian court. A court this week set the next session for March 19.

(Writing by Andrew Hammond; editing by David Stamp)