Arvid Pelshe, 84, the oldest member of the Soviet leadership who was a member of the Petrograd Soviet that played a key role in the 1917 coup in which the Communists seized control of Russia, died May 29. He had lung cancer.

He had been seriously ill for more than a year and appeared in public infrequently. His last public function was on April 22, when he attended a Kremlin fete for the anniversary of Lenin's death.

Mr. Pelshe is the only man in the leadership who had joined the Communist Party before the 1917 revolution and who knew Lenin.

His death created yet another vacancy in the Kremlin hierarchy, giving Soviet leader Yuri Andropov an opportunity to promote new faces into the ruling Politburo. It is expected here that Andropov will make several appointments to the Politburo at the forthcoming plenum of the Soviet Central Committee, scheduled for June 14 and 15.

While his death creates an opening for younger men to the top leadership, it at the same time creates a delicate problem for Andropov. Mr. Pelshe, a Latvian, was the only representative of the three Baltic republics in the top leadership.

Elected to the Politburo in 1966, he was simultaneously appointed head of the party's watchdog control commission, one of the three organs of authority in the Communist Party. As such, he was the most visible representative of smaller nationalities in the leadership dominated by ethnic Russians.

Mr. Pelshe's death reduced to 11 the number of Politburo members. It also symbolized a change of generations that involved the departure from the political stage of Leonid Brezhnev, Mikhail Suslov and Andrei Kirilenko during the past 18 months.

So far, there has been only one addition to the Politburo in this period. Gaidar Aliyev, former security and later party chief of Soviet Azerbaijan, was promoted to the ruling body shortly after Brezhnev died last November.

Mr. Pelshe, who was born Feb. 7, 1899, into a peasant family near Riga, Latvia, joined the Communist Party in 1915. After the 1917 Revolution, he briefly worked in the Cheka--now the KGB--before joining the Red Army, where he served as a political commissar.

After World War II, he first rose to power in his native Latvia, where he became the party leader in 1959 after a campaign against nationalism. A fervent opponent of nationalism, he had written several books on the subject. In his writings he was especially critical of German influences in Latvia.

Since his appointment to the top leadership in 1966, he had continued to deal with nationality problems. However, under his leadership the party control commission had become almost dormant. The commission is supposed to oversee internal party discipline. It also serves as the highest court of appeals for various internal party disputes.