BANGKOK, Thailand (AP) _ Vietnam's Premier Pham Hung, an ''old guard'' revolutionary who ranked No. 2 in the Communist Party, has died of a sudden heart attack. He was 75.

Hung died Thursday afternoon in Vietnam, a Vietnamese Embassy spokesman in Bangkok said today.

The Council of Ministers today named a vice premier, Vo Van Kiet, as acting premier until a permanent succeesor is chosen by the national assembly. Kiet, in his late 70s, is a leading proponent of economic reforms the party says is its No. 1 priority.

A communique read over domestic Vietnamese radio called for three days of national mourning, from Sunday to Tuesday.

Hung died ''while he was leading work in Ho Chi Minh City (formerly Saigon) and other southern provinces,'' said the radio, monitored in Bangkok.

''In the last minute of his life, Comrade Pham Hung ... has closed his eyes like a combatant who has fallen in the battlefield,'' the radio said.

Hung was born on June 11, 1912 in Vinh Long province, now called Cuu Long province, in the Mekong River Delta of southern Vietnam.

The communique said Hung joined the revolution at the age of 16. His radical activities led to his expulsion from secondary school.

He joined the Indochinese Communist Party in 1930, the year it was founded by Ho Chi Minh. The following year, Hung was arrested by French authorities and sentenced to death.

The penalty was commuted to hard labor for life and he was sent to the notorious prison island of Poulo Condor.

Hung was released when communist forces came to power in North Vietnam in 1945 and spent the next nine years of the war against the French in key party positions in South Vietnam.

After the defeat of the French at the battle of Dien Bien Phu, Hung rose rapidly in party and government ranks and returned to North Vietnam. He entered the Politburo in 1956 and two years later became vice premier of the North Vietnamese government.

Hung was a key figure in the war against the U.S.-backed government of South Vietnam. He was sent south in 1967 as secretary of the South Vietnam Commission of the party's central committee and as political commissar of the South Vietnam Liberation Armed Forces, commonly known as the Viet Cong.

In 1975, he served as the political commissar of the Ho Chi Minh Campaign, the final North Vietnamese offensive which climaxed with the capture of Saigon and the end of the long, bloody war.

After 1975, Hung maintained a lower profile than some of his more colorful and outspoken colleagues, but remained in the top ranks of the Communist party hierarchy. He was named a vice premier in 1976, and in 1980 was named Interior Minister, a post that deals with internal security.

At the December 1986 sixth national party congress, he moved up to the No. 2 slot in the party Politburo, behind the newly-chosen party general secretary, Nguyen Van Linh. Three top party leaders resigned at the congress and were replaced by officials regarded as more pragmatic in handling the country's ailing economy.

In June 1987, the national assembly named Hung to replace Pham Van Dong, a veteran diplomat who had held the premiership since 1955. Dong had retired from his party post at the party congress.

Given his age and conservative reputation, Western diplomats said Hung probably was elevated as a ''transitional'' figure who would pave the way for younger technocrats to preside over economic reforms.