Three American activists attempting to publicize a reward for the safe return of American prisoners of war they believe are held in Indochina were arrested on a technicality by Thai police, fined and released, police said Tuesday.

"We charged them as foreigners changing residence without informing the authorities," said Lt. Komsan Suerin, a police investigator in Na-korn Phanom district, 375 miles northeast of Bangkok on the border with Laos.Komsan said the arrests Monday came on instructions from Bangkok to stop the three Americans from releasing reward notices into the Mekong River, which forms the border between Thailand and Laos.

Police identified the three American MIA activists as Amos R. Town-send, 51, of Manchester, N.H., Michael J. Bates, 38, from Ohio, and George R. Handyside, 33, of Atlanta.

They were fined $80 each for violation of a rarely enforced regulation on foreigners residing in Thailand.