Opposition leader Sam Rainsy has issued a letter to Interior Minister Sar Kheng, demanding police release a member of his party, Meach Sovannara, who has been in jail since his arrest last week.
Meach Sovannara is accused of taking orchestrating violent demonstrations in July, a charge he denies. Police arrested him on a warrant for the July 15 protests last week.
Kem Sokha, vice president of the Cambodia National Rescue Party, led a delegation to visit him in prison on Monday.
He told reporters afterwards he expects Meach Sovannara to be released soon.
Police and Interior officials could not immediately be reached for comment.