President Tran Dai Quang of Vietnam, a former police general who presided over a crackdown on free speech in that one-party state, died on Friday. He was 61.

The state-controlled Vietnam News Agency said the death resulted from an unspecified “serious illness.”

Nguyen Quoc Trieu, a government doctor, was quoted by the state news media as saying that Mr. Quang had died of “a rare and serious viral disease” at a military hospital in Hanoi, the capital. Mr. Quang had fallen ill last July and traveled to Japan six times for treatment, Dr. Trieu said.

Vietnam, like China, is governed by an authoritarian Communist Party but promotes a version of state capitalism. Unlike China, where Xi Jinping is both president and Communist Party chief, Vietnam has a power structure in which responsibilities at the top are split among a party chief; a president, who serves as head of state; and a prime minister, who runs the government.