A Cambodian court was due on Thursday to start the treason trial of more than 100 members and backers of the political opposition, in a case condemned by activists as a move by long-serving premier Hun Sen to crush his rivals.

A total of 121 defendants, all tied to the dissolved Cambodia National Rescue Party (CNRP), were summoned to appear on Thursday but many have fled into exile, convinced they would not get a fair hearing...