Police officers allegedly tortured 21 Muslim men during an investigation into a series of bombings in southern India, an international rights group said Tuesday, calling for the officers to be prosecuted.

Bomb attacks in Hyderabad, the capital of Andhra Pradesh state, in May and August last year killed more than 50 people. At the time police blamed Islamic militants and rounded up more than 100 Muslim men.

New York-based Human Rights Watch said that while most were released, at least 21 were held for long periods and allegedly tortured while under detention.

"The detainees were stripped, hung upside down, severely beaten, subjected to electric shocks, and otherwise ill-treated," the group said in a statement. "They were also threatened with the torture of their relatives, particularly female relatives."

The rights group said evidence of the torture was gathered from the testimony of the men and the findings of the state Minorities Commission that concluded that several of them sustained injuries while in custody.

So far no action has been taken against the officers.

The state's chief minister said they would examine the charges.

"Whatever had to be done has been done by the police. If there were any excesses on the part of the police, we will definitely look into it," Y.S. Rajasekhara Reddy said.

Human Rights Watch said it was not an isolated incident.

"Over and over again, the police response to terrible bombings has been to round up people, simply because they happen to be Muslim, and to torture them in the hope of securing information or confessions," said Meenakshi Ganguly, of the group's South Asian office.