A statement issued Tuesday on behalf of 61 Thai nongovernmental organizations said Sombath Somphone disappeared Saturday afternoon in the Lao capital, Vientiane, where friends last saw him getting into his car to drive home from the development agency he founded.
The United States also voiced concern about Sombath's disappearance. State Department spokeswoman Victoria Nuland said the U.S. is urging the Lao government to make effort to locate the activist.
Laos has an authoritarian government with little tolerance for dissent, but friends and associates said Sombath's work was not directly political.
"He deals with business and education. His work isn't the type that would have created enemies," said Suntaree Hathi Sengging of the Thai NGO Coordinating Committee on Development.
An official who answered the phone Wednesday at the government's press office in Vientiane said "It's too early to give any information regarding the disappearance because there is not enough evidence to reach a conclusion." The official declined to give his name.
One of Sombath's colleague in Vientiane told The Associated Press in a telephone interview on Tuesday that he had seen video showing the activist in the custody of police in Laos. He spoke on condition of anonymity because he feared for his safety.
The man said Sombath's relatives on Monday reviewed closed-circuit television footage that showed that at 6 p.m. Saturday, Sombath was stopped by police on his way home and brought to a police station in Vientiane's Sisattanak district. Later, a man walked out of the station with Sombath and drove away with him in a white pickup truck, the colleague said. That was the last known sighting of the activist.
The colleague did not explain where the video footage came from or under what circumstances it was reviewed. He said Sombath's relatives have filed complaints with the police, who he said denied knowing anything about the incident.
Sombath, 60, received the Ramon Magsaysay Award for Community Leadership, one of Asia's top civil honors, in 2005. He was director until five months ago of the Participatory Development Training Centre, which he founded in 1996 to promote education and leadership skills. He is also involved in a small enterprise selling village handicrafts.
The statement from the Thai organizations said they had sent a letter of concern to senior officials in Laos and embassies there. The groups said they "look forward to hearing that all immediate and necessary efforts are made to search his whereabouts and investigate the cause of his disappearance."
The latest U.S. State Department human rights report, for 2011, described Laos as an authoritarian state under one-party communist rule, and said that arbitrary arrests and detentions persisted in Laos despite laws prohibiting them.
Associated Press reporters Elisa Mala and Vee Intarakratug in Bangkok, and Matthew Pennington in Washington contributed to this report.