The Bahraini authorities used "excessive force" during a crackdown on protests earlier this year, an independent commission has said.
Presenting a much awaited report into the events in February and March, the commission said a number of detained protesters had been tortured.
More than 40 people died in the unrest.
Responding to the criticism in the report, King Hamad vowed to do everything possible so "those painful events won't be repeated".
Bahrain has a majority Shia Muslim population and the violence has fuelled anger against the ruling Sunni royal family and political elite.
More than 1,600 people have been arrested during the protests, which have continued sporadically since the peak of the unrest eight months ago.