PRAGUE, Czechoslovakia (AP) _ Vasil Bilak, a member of the old guard that supported the Soviet-led invasion of 1968, resigned from the Politburo and his other Communist Party jobs Thursday, the party announced.

Another Politburo member, Josef Kempny, also stepped down.

Bilak, 71, is a party ideologist linked to the late Soviet leader Leonid I. Brezhnev and was believed to oppose economic reforms favored by Lubomir Strougal, the premier who resigned in October.

He was an associate of former party chief Gustav Husak, now the figurehead president, and helped purge about 500,000 party members after the invasion ended the liberal ''Prague spring'' in August 1968.

The two resignations and the demotion of Central Committee secretary Mikulas Beno were announced at the beginning of a two-day plenum of the party Central Committee.

Two Czechoslovak human rights activists, in a report sent to authorities in May, said Bilak helped prepare the Warsaw Pact invasion and accused him of treason.

Bilak and two others were said to have urged the Soviets to intervene against the reforms set in motion by Alexander Dubcek, then the party chief. The others were the late Drahomir Kolder and Alois Indra, who remains a Politburo member.

In his letter of resignation, Bilak also gave up his posts as Central Committee secretary and head of the committee's foreign policy commission. Jan Fojtik replaced Bilak as chief party ideologist earlier this year.

Party chief Milos Jakes praised the two resigning Politburo members for their party work.

Addressing the plenum, he said Bilak's ''courage ... faithfulness to the party, to the ideals of Marxism-Leninism (and) friendship with the Soviet Union were fully proved in the difficult crisis years and during the overcoming of their consequences.''

The Central Committee is to discuss economic results in 1988 and plans for next year.