Soviet leader Mikhail Gorbachev took over the presidency Saturday, his latest stroke in a two-day shakeup of the government and ruling Communist Party that cleared the way for radical economic and social reforms.Gorbachev, 57, accepted the title of President of the Presidium of the Supreme Soviet, the rubber-stamp parliament, after being unanimously elected by the 1,500-member body in a brief one-hour session hastily arranged on Wednesday.

In his first speech as president, Gorbachev said his program of perestroika had entered a crucial phase and that, under his leadership, the previously rubber-stamp parliament would have a more active role in Soviet domestic and foreign policy.

The Supreme Soviet also named a new KGB chief, appointing deputy KGB chief Vladimir Kryuchkov as head of the secret police agency to replace Viktor Chebrikov, who was seen as an opponent of reform.

Although Chebrikov retained his full-voting status in the Politburo, his reassignment as head of a party committee on legal affairs left the KGB with no voice in the Politburo.

Andrei Gromyko, 79, who was retired from the Politburo Friday, stepped down from the presidency and Pyotr Demichev, axed in the shuffle, was relieved of his parliamentary duties as vice president, the Tass news agency said.

By assuming the presidency, Gorbachev eliminates the awkward position of being Soviet leader but not the official head of state. The presidency is a governmental post and technically not a party political appointment, but the election of Gorbachev gives him ultimate control over every phase of Soviet life.

Although the presidency now is only a ceremonial post, reforms to be implemented next spring would give the presidency wider responsibilities and eventually make it more powerful than Gorbachev's current post of General Secretary of the Communist Party.

Gorbachev, in remarks to the plenary session of the Supreme Soviet, said he would continue to "apply every effort to effect the policy of perestroika, or restructuring," Tass said.