Lim Kimya, 74, who had arrived in Thailand just hours earlier, was shot dead on Tuesday by a waiting gunman who fired three shots, said a police officer, who declined to be named.
Lim Kimya, a Cambodian and French citizen, had crossed into Thailand on Tuesday with his wife and brother and travelled to Bangkok by bus before he was shot dead, the official said.
Police Colonel Sanong Saengmanee, superintendent of the local precinct, told Reuters an arrest warrant had been issued for a 41-year-old motorcycle taxi driver for premeditated murder.
Bangkok police chief, General Siam Boonsom, said an investigation so far indicated the killer was a hit man.
"We have gathered evidence and know who is the perpetrator ... Right now we are working to arrest him," he told reporters.
Lim Kimya was a member of the Cambodia National Rescue Party, the popular opposition that was dissolved by a court ahead of a 2018 election over an alleged treason plot. The CNRP said at the time that the charges against it were fabricated by the ruling Cambodian People's Party.
"Thai authorities should promptly and thoroughly investigate and prosecute those responsible," Bryony Lau, deputy Asia director at Human Rights Watch said in a statement.
"The Cambodian government has intimidated, surveilled and harassed former CNRP members – including those living in exile in neighbouring Thailand."
Cambodian government spokesperson Pen Bona said the killing was a matter for the authorities of Thailand, a sovereign state. He said the Cambodian government was often accused by opposition "extremists" of being behind incidents.
