Rights groups say the strongman is trying to keep his opponents under pressure ahead of local elections this year and a general election in 2018.

Hun Sen filed the latest defamation case against top foe Rainsy for accusing him of bribery in a speech several weeks ago.

“I am waiting for the verdict to be finalised. This time, I will take money ... I am demanding US$1,000,000,” the premier said during a parliamentary session on Tuesday.

“I heard the party’s headquarters is registered under the name of Sam Rainsy, so the party’s headquarters will be auctioned,” he told the assembly, adding that the politician’s personal property would also be “frozen ... and sold”.

The money would be used to build houses for handicapped people, he added.

Rainsy, who currently lives in France and faces several lawsuits, was sentenced to five years in prison in December over a post on his Facebook page, a conviction that makes his return from self-imposed exile unlikely.

On Tuesday Hun Sen also proposed amending a political party law to bar convicts from serving as party leaders – a clear dig at Rainsy and his Cambodia National Rescue Party (CNRP).

His ruling party also voted to strip Rainsy’s deputy Kem Sokha from having the title of minority leader.

Speaking to reporters on Tuesday, party whip Son Chhay sought to brush off the latest moves against the CNRP. “By doing such a thing he will make people unhappy and more and more people will support CNRP,” he said.

Hun Sen has amassed extensive control over Cambodia’s government and economy during his three decades in power and has a history of ruthlessly undercutting his rivals.

He claims to have brought much needed peace and stability to a nation ravaged by civil war and the genocidal Khmer Rouge regime.

But opposition groups have drawn growing support in recent years amid disillusionment with the endemic corruption and rights abuses that have flourished under his watch.

An investigation by Global Witness last year accused the premier and his relatives of carving a $200 million business empire out of the impoverished country’s economy to buttress their political power.

The government dismissed the report but did not directly address the specific allegations detailed by researchers. Rainsy’s opposition party made huge gains in the 2013 elections and say they only lost because the vote was stolen – a claim Hun Sen has vigorously rejected.