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

Human Rights Watch (HRW) last week expressed alarm after the arrests of "dozens of people" since the US-Israeli offensive against Iran began on February 28.

Some of those detained were accused of treason, while others were arrested for protesting.

The arrests also come at a time when Gulf countries are concerned about Iranian infiltration as they come under attack, with Qatar, Kuwait and the UAE all cracking down on what they call Iran-related networks.

On Friday, the Bahrain Institute for Rights and Democracy (BIRD) in the UK said 32-year-old Mohamed Almosawi was arrested at a checkpoint on March 19, and his family had had no news of him since then.

BIRD said that on Friday, his relatives were told to collect his body from a military hospital.

The group published photographs showing injuries to Almosawi's face, body and feet, saying these indicate he may have been tortured before he died.

HRW Bahrain researcher Niku Jafarnia told AFP on Friday she had "received reports from several credible sources today" on the death in custody of Almosawi.

Videos posted by activists from his funeral later Friday in Muharraq in north Bahrain showed a large crowd of angry men following his coffin.

Almosawi had previously spent 12 years in prison, like hundreds of protesters jailed during pro-democracy demonstrations inspired by the Arab Spring in 2011.

He was among around 1,500 prisoners pardoned and released in 2024.