The jailings come as political tensions in Cambodia soar with the opposition party embroiled in a sex scandal and strongman Prime Minister Hun Sen accused of carrying out a fresh crackdown against critics.
Three youth members of the Cambodian National Rescue Party (CNRP) were found guilty of insurrection by the Phnom Penh Municipal court today
The three men were arrested last year after another 11 opposition officials and activists were jailed on the same charges-including three who received 20-year sentences.
Dozens of security officers and several protesters were injured in the clashes.
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At the time the park was a rallying point for Hun Sen's opponents who accused him of rigging the previous year's elections.
The CNRP are currently involved in a fresh standoff with the authorities.
Secret phone calls allegedly between the party's deputy leader Kem Sokha and a mistress were released earlier this year.
But he has refused to be interviewed, arguing that the police investigation is politically motivated.
He has holed himself up in the CNRP's headquarter since late May after armed police last month tried to arrest him.
Hun Sen's government has denied targeting Kem Sokha but has gone on to arrest dozens of critics in recent weeks.
Among them are human rights defenders, environmental campaigners, monks, students, opposition party activists and politicians.
Am Sam Ath, of local rights group Licadho, said today's jailing would fuel further political resentment.