By Agence France-Presse
The damning assessment comes as Cambodia plans to hold nationwide polls next year in what some have warned could be the country's last chance of seeing genuine democracy take root.
Cambodia has been ruled by strongman premier Hun Sen for more than three decades. His reign has brought stability and growth but been criticised as corrupt and autocratic.
The country's once fractured opposition took many by surprise in 2013 when it united to win 55 seats in parliament, an unprecedented move that rattled Hun Sen, a man unused to losing at the ballot box.
At a press conference in Bangkok on Monday, regional lawmakers said Hun Sen's administration has been hitting back ahead of the 2018 polls with measures to cripple the opposition's ability to contest his party.
Asean Parliamentarians for Human Rights, a group made up of former and serving Southeast Asian lawmakers, said Hun Sen has "created a climate of fear, which casts a dark shadow over all of Cambodian society" adding that there was "an ongoing assault on parliamentary democracy".
Recent examples they cited included multiple opposition parliamentarians either jailed or facing court proceedings; recent legislation making it easier to dissolve opposition parties; physical attacks on lawmakers by members of the security forces and the ongoing detention of rights workers.
"Cambodians are facing grave threats to their fragile democratic institutions," Filipino lawmaker Tomasito Villarin told reporters in Bangkok, adding that court cases or the threat of legal action was used "like a Damocles sword" to stifle opponents.
Charles Santiago, a serving lawmaker in Malaysia, said attacks on the political opposition in Cambodia were part of "a new disease sweeping across Southeast Asia".
Examples he cited included recent sedition charges brought against lawmakers in Malaysia and the arrest of a prominent senator and government critic in the Philippines.
Former Thai lawmaker Kraisak Choonhaven said his own country had seen a similar descent into autocracy since the military's 2014 coup.
"We are being buried alive under authoritarian laws against democracy which each day are piling up higher and higher," he told reporters.