Soviet leader Mikhail Gorbachev today attacked President Reagan's response to his recent proposal for worldwide disarmament as he convened the 27th Congress of the Soviet Communist Party, his first since he came to power a year ago.

In a 5 1/2-hour speech, Gorbachev also called for "radical reform" in the economy, said that the Soviet Union and Afghanistan have reached agreement on a conditional schedule for "phased withdrawal" of Soviet troops, and touched on the "damage" done to Soviet society by mismanagement during the rule of Leonid Brezhnev.

It was the 54-year-old Soviet leader's first address to a party congress, the major, long-range policy-setting meeting of the Communist Party that normally takes place every five years. The last congress, in 1981, was presided over by an enfeebled Brezhnev.

After joining the crowded Kremlin Palace of Congresses in silent tribute to predecessors Brezhnev, Yuri Andropov and Konstantin Chernenko, Gorbachev used the occasion -- the 30th anniversary of Nikita Khrushchev's denunciation of Joseph Stalin's reign of terror -- to deliver his vision of the Soviet Union's path into the 21st century and to broadly define his plans for recovering the ailing Soviet economy and morale.

Gorbachev made it clear that he was affirming the stamp he has put on the party in the past year, referring repeatedly to the "April plenum" -- the first meeting of the party's Central Committee held after he came to power. In it the planning for the nine-day congress that opened today was discussed in detail.

The 5,000 Soviet delegates, joined by 154 foreign Communist and Socialist Party delegations, interrupted Gorbachev nearly 60 times with applause and gave him a standing ovation.

Gorbachev defended the Soviet Union's proposal on Jan. 15 for a three-stage elimination of nuclear ar w0080 ----- r a BC-02/26/86-GORB 1stadd w0080 02-26 A01 NATO fellow travelers made a steep turn from detente to a policy of military force."

But now, he continued, "in U.S.-Soviet relations there seem to be signs of a change for the better, and realistic trends are beginning to emerge in the actions and attitudes of the leadership of some NATO nations."

In his discussion of the Soviet economy, Gorbachev said that "a radical reform is needed."

He proposed several new programs for assisting economic recovery, including linking an enterprise's payroll to the return on its sales, pumping 200 billion rubles ($270 billion) into the Soviet machine-building industry from now until 1990, and a "widespread" system of contracting agricultural production work to "teams, groups and families."

Hardening his earlier attacks on outmoded economic approaches, Gorbachev said that "every readjustment of the economic mechanism begins . . . with a readjustment of thinking, with a rejection of old stereotypes and actions."

But the Soviet leader fell short of explicating a grand scheme for achieving economic reform or filling in the blanks in his program for social and economic development until the year 2000, to be presented later in the congress. He stressed instead reliance on established methods such as a productivity increase for achieving a two-fold increase in industrial output by 2000.

In an apparent departure from the strict ideological policies of his predecessors, Gorbachev told the audience of Soviet, East Bloc and other Communist and Socialist representatives that "complete unanimity among Communist parties does not exist always and in everything. . . . We do not see the diversity of our movement as a synonym for disunity."

He effusively praised the new-found ability of Moscow and Peking to "cooperate on an equal and principled basis."

Since Gorbachev came to power, the ongoing Sino-Soviet thaw has accelerated. But China declined an invitation to come to the congress, citing the fact that there are no Sino-Soviet party-to-party relations.

Without naming Brezhnev, who died in 1982, Gorbachev said that mismanagement and bureaucratic inertia in the 1970s had "inflicted no small damage to our society and our cause." He said that "the situation called for change, but a peculiar psychology -- how to improve things without changing anything -- took the upper hand in the central bodies and, for that matter, at the local level as well."

Gorbachev spoke on a wide range of themes, including the following:

*The KGB, the Soviet secret police: "Greater responsibility devolves on the state security bodies. We are convinced that Soviet security forces will always display vigilance, self-control and tenacity in the struggle against any encroachment on our security system."

*On cultural unions: "The main result of their work is measured not by resolutions and meetings, but by talented and imaginative books, films, plays, paintings and music needed by society and which can enrich the people's intellectual life."

*On the party's image: "We bear quite a lot of damage because some Communists behave unworthily or perpetrate discrediting acts. . . . The party will go on resolutely getting rid of all who discredit the name of a Communist."