New Delhi, Aug. 25 -- An amnesty for political offenses committed in Thailand over the past two decades took effect Monday, but the law excludes some of the more serious cases, including those involving lese majeste, or defaming the monarchy.

The amnesty was introduced to promote reconciliation after past political instability, violent protests and military coups and could cover about 6,000 people, according to estimates made in Parliament. But the exclusion of lese majeste offenses has drawn criticism that the amnesty will not truly resolve political divisions.

The Peaceful Society Promotion Act passed a final vote by lawmakers in July and was published in the Royal Gazette on Sunday. A committee chaired by the prime minister will dete...