The verdict was widely expected from a justice system heavily warped by the influence of long-standing premier Hun Sen, who is accused of ruthlessly targeting rivals in the run-up to 2018 polls.
It nevertheless delivered a crushing blow to what remained of the embattled Cambodia National Rescue Party (CNRP) -- the only party that had a fighting chance to break the authoritarian leader's 32-year grip on power.
The judge said the CNRP, by boycotting the trial, had effectively confessed to the government-levied accusation of conspiring with the United States and other foreign actors to plot a revolution.
The CNRP and Washington have rejected those charges as bogus, with the main evidence from the government being a publicly available speech from the party's president discussing US help to build a democracy-movement in Cambodia.
"This is the death of democracy in Cambodia," said Phil Robertson from Human Rights Watch, calling on for foreign partners to suspend any assistance for the 2018 poll.
The International Commission of Jurists said Cambodia had crossed a "red-line," with the dissolution stripping millions of voters of a chance to freely choose their representatives.
The CNRP's parliament seats and local posts will now be redistributed to other parties after the government amended laws last month to allow the reallocation.
That Cambodia towards being a de facto one party state, with it highly unlikely opposition activists can now mount any significant challenge to the CPP next year.
The crackdown accelerated dramatically after the party faired well in local elections this July, despite weathering a series of legal attacks against its leadership.
Several months later CNRP leader Kem Sokha was suddenly thrown into jail charged with treason over the same accusations of plotting a revolution.
That arrest, and the threat of a 30 year prison sentence, sent more than half of the party's 55 lawmakers fleeing into exile out of fear.
But observers say the current climate of repression is harsher and longer-lasting than previous clampdowns, with Hun Sen foregoing even the pretense of respecting human rights and a free press.
In addition to assaults against the CNRP, his government has in recent months shut down a series of outspoken NGOs and independent news outlets -- including the respected Cambodia Daily.
Analysts say the premier has been emboldened by financial backing from Beijing, which has lavished the poor country with investment that has made it less dependent on aid from Western democracies.
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