BEIRUT, Lebanon — Security forces in Bahrain used excessive force, including torture and the extraction of forced confessions, against detainees who were arrested in a sweeping crackdown early this year during protests that deeply polarized the country, according to a report by an independent commission that investigated the uprising and its aftermath.

The report, released on Wednesday, presented a devastating portrait of what it called disproportionate and indiscriminate force often used by the security forces to repress protests in February and March that were organized primarily by the Shiite Muslim majority in Bahrain, a tiny Persian Gulf state that is a prominent American ally.

“A number of detainees were tortured,” M. Cherif Bassiouni, an international law expert who led the inquiry, said at a news conference in Manama, the capital, as King Hamad bin Isa Al Khalifa listened. Mr. Bassiouni added that the dimensions of the torture “proved there was a deliberate practice by some.”

The protesters began by calling for a more equitable political system, including a constitutional monarchy. But as the violence escalated, some demonstrators demanded the dismantling of the political structure, and the Sunni monarchy at times appeared to fear that the family that had conquered Bahrain in the 18th century was about to be overthrown.