Marshal Dmitri Ustinov, the Soviet defense minister and one of the most powerful Kremlin figures in recent years, died last night at age 76, it was announced here today.

Ustinov's departure leaves a major gap in the ranks of the Soviet leadership, underscoring a painful generational change that has been under way for the past two years. His successor was not immediately announced, but observers expected no changes in Soviet foreign or security policies.

An official communique said that Ustinov's condition had deteriorated steadily following an operation on the aorta, the main artery carrying blood from the heart, and that death resulted from cardiac arrest. The surgery to correct a serious circulatory problem was carried out while Ustinov was recovering from pneumonia, for which he was hospitalized in late October.

Ustinov's death removes one of the old guard members of President Konstantin Chernenko's Politburo and a figure of major standing in the Communist Party elite. Ustinov had played one of the major roles in the last two Kremlin successions.

It was not known whether the defense post would be turned over to another civilian figure or whether a professional soldier would be tapped for the job.

Western diplomats saw a possible hint in the selection of Politburo member Grigori Romanov, 61, as head of the committee organizing Ustinov's funeral.

Ustinov, a professional engineer deeply involved in managing the Soviet arms industries for most of his career, had headed the committee that organized the funeral of his predecessor as defense minister, Marshal Andrei Grechko, in 1976. Ustinov was appointed defense minister immediately after Grechko's funeral and was promoted to the rank of marshal three months later.

Marshal Sergei Sokolov, 73, who has been serving as first deputy defense minister since 1967, was seen as a possible candidate if the leadership decides to name a professional soldier to the post. Sokolov has been acting for Ustinov in his absence since late October.

The first indications that a senior Kremlin figure had died came early today when a postponement of the world chess championship was announced. The championship matches are held at the Hall of Columns, an 18th century building in central Moscow where top officials lie in state before state funerals.

Turkish officials reported later that Premier Nikolai Tikhonov, who was scheduled to visit Ankara on Monday, had put off the start of his visit by one day.

The announcement that Ustinov had died came later in the day from Politburo member Mikhail Gorbachev, 53, the second-ranking figure in the party hierarchy, who cut short his visit to Britain and flew back to Moscow.

Whether Ustinov's departure would significantly affect the balance of forces in the Politburo was not immediately apparent. Should this happen, observers here speculated, it was more likely that it would affect domestic rather than foreign policy.

One recent personnel change, involving the demotion of Marshal Nikolai Ogarkov, the former chief of staff, was interpreted here as an effort by the party leaders to assert their supremacy in military affairs.

Before he was suddenly removed last September, Ogarkov had been widely regarded as Ustinov's eventual successor. The 66-year-old Ogarkov is one of the most impressive and articulate officers in the Soviet Union and has been a forceful advocate of military interests.

By comparison, it was never quite clear whether Ustinov was the armed forces' man in the Politburo or the Politburo's man running the military establishment.

Ustinov, who became minister of armaments at age 32, has been at the top of Moscow's military-industrial complex for more than 40 years. He became one of the principal Politburo figures under Brezhnev, a role expanded still further under the late president Yuri Andropov and, more recently, Chernenko.

While Andropov was ailing in late 1983 and early 1984, Ustinov became the Kremlin's most forceful and visible spokesman. When Chernenko assumed the party leadership in February, Ustinov and Foreign Minister Andrei Gromyko appeared to be the strongest figures in the new leadership.

Because of his prestige and experience, Ustinov had been viewed here as a "kingmaker" since the death of Politburo member Mikhail Suslov in January 1982. As such, Ustinov was said to have exerted a steadying influence during the two Kremlin transitions.

His large frame and bespectacled face were familiar to the Soviet public, and he was widely admired for his physical stamina in taking the military salute for hours during the annual Nov. 7 Red Square parades. His failure to appear on the reviewing stand last Nov. 7 was the first indication that Ustinov was seriously ill.

Ustinov will lie in state at the Hall of Columns Saturday and Sunday before he is given a state funeral on Monday and buried in the Kremlin wall.