PHNOM PENH (Reuters) - Cambodia could make more arrests of people linked to a plot to overthrow the government, Prime Minister Hun Sen said on Monday, attacking opposition figures as  rebels in the city  bent on staging a  color revolution , despite past failures. The comment is part of the strongman s widening crackdown on opposition politicians and independent media that critics and rights groups see as an effort to shore up his party s strength ahead of a general election next year. Last month, authorities arrested Kem Sokha, leader of the opposition Cambodia National Rescue Party (CNRP), and charged him with treason and espionage over an alleged plot to take power with help from the United States. The U.S. embassy has rejected any suggestion of interference in politics. The Sept. 3 arrest sparked condemnation from Western countries, and the CNRP has said about half its members of parliament have fled Cambodia fearing a crackdown by Hun Sen.  This isn t over yet with one arrest, I would like to send out a message,  Hun Sen, who has ruled Cambodia for more than 30 years, said at an opening ceremony for a hotel in Siem Reap province.  This set-up is a systematic activity,  he added, referring to the alleged opposition plot to overthrow his government.  The rebels in the city that do  color revolution  are absolutely rejected.  Hun Sen can claim support from his main ally, China, which has said it supports Cambodia s efforts to maintain its security. The government says a 2013 video in which Kem Sokha tells supporters he has help from unidentified Americans to gain power is evidence that he was colluding with the United States. The opposition argues the video is evidence of an election strategy, not a coup plot. 