BANGKOK, Thailand (AP) _ Truong Chinh, one of the last of Vietnam's Old Guard revolutionaries and former head of its Communist Party, has died, the Vietnam News Agency said today. He was 81.

The official news agency said Chinh died at his Hanoi home on Friday of severe hemorrhaging about an hour after an accidental fall. Five days of official mourning will begin Sunday.

Chinh belonged to the passing generation of leaders who founded the Communist Party in Vietnam, fought four wars and united the country in 1975.

A hard-line ideologue, Chinh was replaced as general secretary of the Communist Party of Vietnam in December 1986 when reformers led by his successor, Nguyen Van Linh, gained the upper hand.

He had since served as a party adviser. Although the official reason for his resignation was old age and declining health, failure to pull Vietnam out of serious economic problems was believed to have caused his exit.

His death leaves only Pham van Dong, Le Duc Tho and Vo Nguyen Giap among a group of near-legendary figures who successfully fought the French, Japanese, Americans and Chinese but failed to bring prosperity to Vietnam.

Communist literature has lauded Chinh as ''the first builder and commander of the Vietnam revolution.''

U.S. government documents describe him as Vietnam's most hated leader, with French historian Bernard Fall estimating 50,000 people were executed in Chinh's drive to ''reform'' the North Vietnamese countryside during the 1950s.

Lacking the charisma of longtime premier Dong or the military mind of the ''Red Napoleon'' Giap, Chinh made his name as a party organizer and Marxist ideologue.

Once regarded as pro-Chinese rather than pro-Soviet, he changed his name of Dang Xuan Khu to Truong Chinh, meaning ''Long March,'' because of his admiration for the epic march of Chinese leader Mao Tse-tung. But some say his pro-Chinese tendencies were exaggerated by oustide analysts.

Except for a brief eclipse after the disasterous land reforms, Chinh remained near the pinnacle of power from the 1930s. He long held the No. 2 spot in the party's Politburo and in 1981 was named chairman of the newly created State Council, the most powerful government organ.

He took over the partty leadership on July 14, 1986, after the death of another top Old Guard revolutionary, Le Duan, but was viewed by some as a transitional figure while Vietnam moved toward more economic reforms and some political liberalization.

According to his official biography, Chinh was born in the northern province of Ha Nam Ninh, began political activities in 1925 and joined the Revolutionary Youth League. In 1930 he became one of the founding members of the Indochinese Communist Party.

Like many of his colleagues, he was imprisoned by the French colonial rulers and at the outbreak of World War II fled to China where he helped organize forces that under the leadership of Ho Chi Minh entered Hanoi at the end of the war and proclaimed the Democratic Republic of Vietnam in 1946.

In the ensuing war against the French, which culminated in their annihilation by Giap's troops at the battle of Dien Bien Phu, Chinh directed the propaganda machine. By 1953, he was second to only ''Uncle Ho'' in the power hierarchy.

He was dropped from his first-time role as Communist Party chief after confessing to ''serious mistakes'' in his brutal drive to collectivize agriculture in the north. But his fall was brief, and in 1960 he was elected head of the National Assembly.

His stature increased as the United States entered the war in South Vietnam. Rather than throwing regular North Vietnamese units into the conflict, he advocated a ''protracted guerrilla war'' and a greater emphasis on political struggle.

A 1969 U.S. government biography of Chinh said, ''Essentially a behind-the- scenes planner, austere and an extremist, Truong Chinh has the inflexible mind of the fanatic to whom only the world of party doctrine has any meaning. Of all the top-ranking leaders in North Vietnam, he is possibly the best versed in communist ideology and certainly the most hated.''

After the 1975 communist victory in South Vietnam and subsequent unification of the country, the record of the Old Guard proved poor. Some Vietnamese officials have admitted the Old Guard ''knew how to run a war but not the economy.''

The situation deteriorated further after Hanoi's 1978 invasion of Cambodia, which prompted most Western nations to minimize contacts with Vietnam.