BANGKOK, Thailand -- Police said Tuesday they shot and killed 42 people during a seven-week-old crackdown on drugs that has drawn protest from human rights groups.

Since the campaign began, police have arrested more than 42,000 drug suspects and seized 12 million amphetamine pills and assets worth more than $12 million, said police spokesman Maj. Gen. Pongsaphat Pongcharoen.

Among those arrested were nearly 400 drug makers and more than 12,000 dealers, Pongsaphat said. The rest were drug users or couriers.

Six police officers were killed and 15 injured during the crackdown, Pongcharoen said.

Human rights groups and the United Nations have expressed concern that many of the killings may have been summary executions carried out by the authorities.