The threat of more legal action against Sam Rainsy, who has lived in France since 2015 to avoid a series of convictions, comes weeks after a court dissolved his opposition party, removing any significant challenge to Hun Sen extending his decades-long rule in a general election next year.
The dissolution of the Cambodia National Rescue Party has been condemned by the opposition, rights groups and some western countries as the most serious blow to democracy since an international peace deal and UN-run elections in the early 1990s ended decades of war and genocide.
Sam Rainsy, who stepped down as leader of the CNRP this year in what turned out to be a futile bid to forestall a ban on his party, suggested in a video posted on his Facebook page on Tuesday that soldiers would not obey orders to shoot civilians.
Hun Sen, who has held power for more than 32 years, said the military would file a lawsuit in response.
The Supreme Court banned the CNRP after its leader, Kem Sokha, who took over after Sam Rainsy stepped down, was arrested for allegedly plotting to overthrow the government.
Kem Sokha, who rejected the accusation, is in prison.