BANGKOK, July 13 (Reuters) - The Thai army said on Friday it had detained 342 Muslims, including seven women, in raids in the rebellious far south, one of the largest series of arrests in more than three years of separatist violence.

Human rights group criticised the move, saying it exposed detainees to potential abuses by the army, which is operating under martial law that grants soldiers immunity from prosecution.

The detainees were taken for questioning to five army camps across the four southern provinces where more than 2,300 people have been killed in a three-year insurgency.

Army spokesman Colonel Acra Tiproch said in a statement the detainees, who can be kept without charge for 28 days under emergency security laws, would be freed if found not to have been involved in the violence.

Samples of their DNA would be kept on record.

Since taking office last year following a bloodless coup, Prime Minister Surayud Chulanont has fought off pressure from Thailand's Buddhist majority to take stronger action in the Muslim, Malay-speaking region, saying he remains committed to a peaceful resolution.

He has apologised for the harsh policies of his ousted predecessor, Thaksin Shinawatra, and promised restraint in dealing with the violence.

But he has had no more success than Thaksin in dampening the violence in the region, which is a former Muslim sultanate annexed by Thailand about a century ago.