Bahrain is ruled by a Sunni Muslim dynasty, but the tiny kingdom has a large Shia population that has long complained of marginalisation.

A Bahraini court "sentenced the defendant to life imprisonment," the kingdom's Special Investigation Unit said in a statement.

Authorities were made aware of the death of the detainee, who was not identified, on March 27, through social media and in a report from the general inspector of the intelligence service that showed he had been beaten by one of its officers.

On the same day, the London-based Bahrain Institute for Rights and Democracy (BIRD) reported the death of Shia activist Mohamed Almosawi, 32, who was arrested at a checkpoint a week earlier.

Authorities said he had been suspected of spying on behalf of Iran.

BIRD said that Almosawi's relatives were told to collect his body from a military hospital and published photographs showing his injuries. It called for an investigation into his death.

Iran's attacks on the Gulf following US and Israeli strikes and the killing of its supreme leader, Ali Khamenei, a spiritual figure for many Shias, have stirred tensions in Bahrain.

Since the war began, Bahrain has cracked down on those expressing support or sympathy for Tehran.

Some have been accused of espionage, and more than 300 mostly Shia Muslims have been arrested, according to Bahraini activists.

Bahrain has denied cracking down on citizens based on their religious identity.