PHNOM PENH, Cambodia — A court on Thursday found the two most senior surviving leaders of the Khmer Rouge, which brutalized Cambodia during the 1970s, guilty of crimes against humanity and sentenced them to life in prison.

The chief judge, Nil Nonn, said the court found that there had been a “widespread and systematic attack against the civilian population of Cambodia” and that the two men had been part of a “joint criminal enterprise” that bore responsibility. They were convicted of murder and extermination, among other crimes.

More than 1.7 million people died under the rule of the Khmer Rouge from 1975 to 1979.

The proceedings of the tribunal, a joint effort of the Cambodian government and the United Nations, have been criticized for being extremely belated and for covering only a narrow sliver of the Khmer Rouge’s crimes. The judgments against the two men — Nuon Chea, 88, and Khieu Samphan, 83 — were the first handed down against the Khmer Rouge leadership, although a lower-ranking official, who ran a notorious prison for the regime in Phnom Penh, was convicted in 2010. Both defendants will appeal, their lawyers said.