Prime Minister Hun Sen, who has held power for nearly three decades, shook hands with opposition leader Sam Rainsy as their first face-to-face talks since last September got under way.
In a message posted on his Facebook page earlier, Rainsy described the meeting as "final talks".
His Cambodia National Rescue Party (CNRP) has boycotted parliament since elections last July, accusing Hun Sen's ruling party of stealing victory with widespread vote-rigging.
Rights groups have denounced the charges against the politicians, who could face up to 30 years in prison if convicted.
The CNRP wants sweeping electoral reform before new polls are held. It has previously rejected Hun Sen's offer to stage fresh elections in February 2018.
"The leaders will discuss issues including the arrest of the opposition politicians, reform of the election body and the (date of) a new general election," a CNRP official told AFP, requesting anonymity.
A crackdown on striking garment workers in January left four people dead.
Experts said last year's poll loss dealt a major blow to the CNRP which was adamant it had won the election.
However, it also represented a significant parliamentary gain on its performance at the previous vote.
Opposition supporters have staged several major demonstrations against Hun Sen's government.
