Son Chhay, a senior member of the opposition Cambodia National Rescue Party (CNRP), made the pledge to challenge the prime minister's ruling Cambodian People's Party (CPP) Tuesday during a news conference in Phnom Penh.
CNRP leader Kem Sokha was taken from his home on September 3 and charged with treason.The government said the charges were based on comments Kem Sokha made on a video dating back to 2013, in which he claims to have received help from the United States to build a pro-democracy movement in Cambodia.
Hun Sen has threatened to dissolve the CNRP if it intervenes on Kem Sokha's behalf.
Sokha's arrest was carried out amid a massive government crackdown against independent news outlets and human rights groups. Last Monday The Cambodia Daily, one of the last independent newspapers in the country, was closed after it received a massive overdue tax bill its publishers claim is bogus.
The crackdown is an apparent attempt by autocratic Prime Minister Hun Sen to shut down dissenting voices ahead of next year's election with the aim of extending his three-decade-old grip on power.Hun Sen's government was nearly toppled in the last national election in 2013, and support is growing for the opposition, especially among younger Cambodians.