PHNOM PENH (Reuters) - Cambodia s detained opposition leader, who is accused of treason by the government of strongman Prime Minister Hun Sen, denied the charges against him on Monday in his first comments since his arrest last month.  Kem Sokha, head of the opposition Cambodia National Rescue Party (CNRP) is accused of trying to overthrow the government with help from the United States and of espionage - charges he denies and says are politically motivated. His arrest last month is part of ongoing efforts by the government to silence political opponents and independent media ahead of a general election next year. Earlier on Monday Hun Sen, a former Khmer Rouge officer who defected and has ruled Cambodia for more than three decades,  attacked opposition figures as  rebels in the city  bent on staging a revolution. In a three-page letter posted on his Facebook page on Monday Kem Sokha said that he was seeking positive change in Cambodia through the ballot box and not through revolution as charged.     As the leader of the CNRP and a representative of more than half of the country s population, I always choose changes through elections without violence,  he said in the letter. Rights groups say Hun Sen, 65, is determined to extend his rule and dismantle the burgeoning popularity of the CNRP, who have gained from public anger over inequality and cronyism in the country under Hun Sen s rule.     The U.S. embassy has rejected any suggestion of interference in politics.  Kem Sokha s 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.    In his letter, Kem Sokha called his arrest unconstitutional and urged supporters not to be intimidated.     Please, the Cambodian people, remember that I am with you. Even though my body is in prison, but nobody can detain my conscience,  he said. He called on the international community for help.   Democracy in Cambodia is already at its last breathe,  he said.    A spokesman from Hun Sen s Cambodian People s Party (CPP) said that Kem Sokha had confessed to the charges against him. 