"Amnesty International is gravely concerned by the recent killing of Charoen Wadaksorn, a well-known human rights defender, and is calling on Thai authorities to take immediate action to investigate his killing and to protect other human rights defenders," the London-based group said in a statement.

It also urged the Thai government "to demonstrate its commitment to protect human rights defenders by ensuring that justice is done and seen to be done in the investigation of this and other killings."

Charoen, 37, was shot dead by two gunmen Monday only hours after he gave evidence to lawmakers about a businessman's plan to build a resort on public land.

He was also prominently involved in a successful campaign to force the government to scrap its proposal to build a coal-fired power plant near his village in Prachuap Khiri Khan province.

Thailand's Prime Minister Thaksin Shinawatra said Wednesday he would "personally monitor" the police investigation into the murder.

Charoen's wife claimed he was killed because he blocked corrupt property deals and demanded an investigation by Thailand's elite Special Investigation Department (SID).

Thaksin rejected the demand for a SID investigation but Interior Minister Bhokin Bhalakula said police must conduct a swift inquiry.

Regional rights group Forum-Asia also condemned the killing, which it said was the 15th murder of a human rights defender in Thailand since Thaksin took office in 2001.

"This crime... is an open war on human rights defenders and a direct challenge by the perpetrators toward both the executive and legislative power of Thailand," Forum-Asia said in a statement.

The Asian Human Rights Commission said 17 rights and development non-governmental organisations (NGOs) will submit a letter to Thaksin urging him to become more involved in the case as local police could be easily influenced by wealthy and powerful figures.

"The NGOs said they will also urge the Thai government to arrest the gunmen and masterminds of Charoen's murder and to establish an independent mechanism to protect human rights defenders," the commission said in a statement.

"Charoen's family and human rights groups believe influential figures are behind his murder."

TERRA.WIRE