PHNOM PENH: Eleven Cambodian opposition members and activists were jailed on insurrection charges Tuesday, including three who received 20-year sentences, a defence lawyer said.

Rights groups said the draconian penalties, for taking part in clashes in July 2014 over the closure of a key protest site in the capital, were imposed as strongman premier Hun Sen intensifies efforts to smother dissent in a kingdom he has led for more than three decades.

Those jailed are from the opposition Cambodian National Rescue Party (CNRP). They were arrested during the clashes with authorities last year that left dozens wounded on both sides.

Tuesday's shock ruling caught lawyers, rights groups and CNRP supporters flat-footed.

"This is a very serious sentencing. We lawyers cannot accept these convictions," Sam Sokong, one of the group's defence counsel told AFP, adding his eight other clients received seven years in jail.

Local rights group LICADHO described the proceedings as a "show trial with a predetermined ending, apparently set up only to intimidate the CNRP" -- adding that the surprise sentencing was caried out without the presence of most of the group's lawyers.

Hun Sen, a master operator within Cambodia's politics, has brooked little dissent during his time in office. He has successfully played off his rivals, deftly using a combination of the courts, hard power and small compromises to blunt opposition.

Last week parliament passed a controversial law with his backing to regulate non-governmental organisations, despite a boycott by opposition lawmakers and street protests.

Rights groups, UN officials and Western diplomats also voiced strong concern over the legislation, which they say will expose campaigners on flashpoint issues such as land grabbing and human rights abuses to prosecution.

Observers said the swiftness and severity of Tuesday's sentences could be a response to the CNRP's boycott of last week's vote and signalled a deep freeze in relations between the two parties after recent improvements in ties.

"There may be a connection with the fact that the opposition walked out of parliament last week when the NGO law went through," Andrea Giorgetta from the International Federation of Human Rights, told AFP.

"This is one of the worst convictions we've seen in recent times in Cambodia. It's clearly disproportionate in relation to the events that took place on that day."

On July 15, 2014, violent protests at the capital's main demonstration site -- Freedom Park -- left dozens of rally-goers and police injured.

The CNRP, which is led by Sam Rainsy, has denied allegations that its members were involved in the violence.

But the insurrection charges that followed came during a protracted political standoff between the ruling Cambodian People's Party (CPP) and the CNRP.

At the time opposition lawmakers were refusing to take their seats in parliament, alleging that a 2013 general election had been rigged.

The CNRP took its seats a week after the charges were laid against the activists, ending a year-long political impasse.

Several of its MPs were also accused of insurrection over the protest.

Their trials have not yet started, although Tuesday's convictions are likely to send a shiver through the party faithful.

The CNRP took up its seats in the National Assembly on July 22 last year in return for a promise of electoral reforms, but it has since been broadly frustrated by Hun Sen's manoeuvring.

Hun Sen has faced mounting criticism from rights groups over the suppression of street protests but maintains he will stay in power until he is 74.

He is currently in his early 60s.