President Mikhail S. Gorbachev's sudden overhaul of the Kremlin power structure reinvigorates his reform drive and signals his determination to best the bureaucracy he blames for slowing him down.

The Soviet leader's swiftly executed power play Friday and Saturday elevated him to the presidency, put him in charge of the government as well as the ruling Communist Party and left the Kremlin with no clear No. 2.In action-packed top-level meetings, Gorbachev practically established a mandatory retirement age by pushing all but one septuagenarian off the party's ruling Politburo and sent an unmistakable message through the vast Soviet bureaucracy that he expects a similar pruning of dead wood at all levels.

At the same time, by easing the 79-year-old President Andrei A. Gromyko into retirement with praise and a televised swan song, he opened the way for Soviet officials to leave office gracefully, a necessary prelude to the institution of fixed terms of office for party and government officials.

The sole survivor from the Old Guard is Vladimir V. Shcherbitsky, 70, whose power base built during 16 years as Ukrainian party boss apparently continues to insulate him from Gorbachev's campaign to replace old blood with new.

The new face in the 12-man Politburo, Vadim A. Medvedev, 59, was catapaulted over eight candidate, or non-voting, Politburo members to full voting status. He was given responsibility for ideology, which by previous custom would make him second to Gorbachev.

Medvedev, an economist regarded by Western analysts as a proponent of radical economic reform and a member of Gorbachev's perestroika "brain trust," will share equal rank with five other party commission chiefs, four of them strong Gorbachev supporters and reformists.

"It was certainly spectacular to jump him into the senior rank," said one senior Western diplomat.