Acting president of the opposition Cambodia National Rescue Party Kem Sokha yesterday visited detained CNRP members at Prey Sar prison.
Fourteen party activists are currently locked up for their roles in anti-government protests in July 2014.
Sam Rainsy Party Senator Hong Sok Hour has also been detained since August, accused of incitement and forgery for posting a "fake" version of a border treaty online.
Kem Monovithya, CNRP’s deputy public affairs head and Sokha’s daughter, said efforts to negotiate the men’s release were ongoing but not currently promising.
"We are in communication with the CPP, but there hasn’t been any positive response from them with regard to the prisoners," she said.
"It’s something they understand we’ve always been asking for, there’s nothing new this week or last week."