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

Those jailed are from the opposition Cambodian National Rescue Party (CNRP) and were arrested during clashes with authorities in July 2014 against the closure of Phnom Penh's main designated protest site.

"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 made without the presence of most of the group's lawyers.

Cambodia has been ruled for 30 years by Prime Minister Hun Sen, a strongman who has tolerated little dissent during his time in office.

The CNRP is the country's main opposition party to Hun Sen's Cambodian People's Party (CPP).

The controversial insurrection charges were made at the end of a protracted political standoff between the CPP and the CNRP after the 2013 election.

At the time opposition lawmakers were refusing to take their seats in parliament alleging that the election had been unfairly won by Hun Sen.

A decision in July 2014 to close the capital's main protest site at Freedom Park sparked violent demonstrations with injuries on both sides.