PHNOM PENH, Cambodia — From behind the lawyers, a hand went up, calling for attention as the trial of the four surviving leaders of the Khmer Rouge got under way Monday on charges of genocide, war crimes and crimes against humanity committed more than three decades ago.

It was Nuon Chea, 84, one of the defendants, bundled against the air-conditioning in a striped knit cap and sheltering himself from the bright lights with a pair of large dark glasses.

“I am not happy with this hearing,” said Mr. Nuon Chea, who is described as the Khmer Rouge’s chief ideologue. Then he rose from his seat and walked unsteadily from the courtroom with the help of three security guards.

As the chief judge noted, the holding cells adjacent to the court have video links and telephone lines, and the defendants are free to choose to participate by video link....