MOSCOW, JULY 13 -- President Mikhail Gorbachev, winding up a landmark Communist Party congress, called today for a political coalition with "other democratic forces" to rescue the Soviet Union from its growing economic and political crisis.

The congress, which was marked by 12 days of open feuding between conservatives and radicals, ended with a tactical triumph for Gorbachev, despite an announcement today by the mayors of Moscow and Leningrad that they will leave the party and the defection of about 50 delegates representing the radical group Democratic Platform. {Details, Page A14.}

The 59-year-old Soviet leader succeeded in winning a centrist majority in the new policy-making Central Committee as well as the departure of his most prominent conservative rival, Yegor Ligachev. Flushed with his success, Gorbachev told the nearly 4,700 delegates that the congress had taken a major step toward renovating the party that has ruled the Soviet Union for 73 years.

"The Communist Party of the Soviet Union lives and will continue to live," he declared to applause from the hall. "Those who had been calculating that this would be the last congress and that it would witness the party's funeral have miscalculated."

Only a tiny proportion of the party's outgoing leadership is likely to be reelected to the Politburo, the body that runs the party on a day-to-day basis. Half the members of the former Politburo have not even been chosen to the broader policy-making Central Committee that serves as a kind of party parliament between congresses.

The new Politburo will be largely made up of representatives of the Soviet Union's 15 republics, reflecting a devolution of some political power away from Moscow. Government officials, such as Foreign Minister Eduard Shevardnadze and Defense Minister Dmitri Yazov, are likely to be excluded from the Politburo in order to emphasize the separation of party and state.

In his final speech to the congress, Gorbachev offered an olive branch to the radicals who have announced their intention to set up a rival political party at a congress in the fall. He said the Communist Party is willing to cooperate with other mass movements and organizations and would "drastically change" its attitude toward the elected soviets, or councils, which until recently were little more than a facade for party rule.

"The idea of a broad coalition that is contained in the decisions of this congress in order to overcome the crisis and carry out profound reform is not a tactical move, but a serious proposal dictated by the interests of the country and the people," he said.

Gorbachev also said the Soviet Union would welcome economic aid from the West but rejected any political conditions. At their summit in Houston this week, leaders of the seven major industrialized democracies called on Moscow to shift resources from the military. {Related story on Page A15.}

After a rocky start, during which conservative delegates sharply attacked the leadership for both its foreign and domestic policies, the congress fell into line behind Gorbachev. He dominated the frequently fractious hall, sending Ligachev into political retirement after an unsuccessful bid to become deputy leader of the party.

"Gorbachev has won a sweeping victory," declared Stanislav Shatalin, a liberal economist who belongs to the Presidential Council. "The congress gradually became more radical and more businesslike as the debates wore on. As a result of this protracted struggle, Gorbachev has now got a complete mandate. He is president and general secretary of the Communist Party. There is no need for him to hide behind the Central Committee or the Politburo any longer."

The key to Gorbachev's success lay in his ability to impose his will on a large group of essentially conservative-minded delegates easily swayed by appeals to party discipline. On several occasions, he reversed decisions he did not like by telling the delegates that they had erred and ordering a new vote.

This evening, in a final attempt to demonstrate its independence, the congress voted to exclude several Gorbachev nominees from the policy-making Central Committee. But the vote was reversed moments later after Gorbachev declared that he would consider the congress a failure unless the mistake was corrected forthwith.

The candidates initially blackballed by the congress included several leading liberals as well as reform-minded party bureaucrats whom Gorbachev undoubtedly sees as his natural allies in the new Central Committee. Deputy Prime Minister Leonid Abalkin, who has been responsible for supervising the planned transition to a market economy, was among several economists to attract large numbers of negative votes.

In the end, the congress decided to expand the Central Committee to 412 full members to allow room for the entire slate of candidates. New members of the Central Commitee include several avowed social democrats, such as Shatalin and liberal playwright Alexander Gelman, and dissident historian Roy Medvedev, who was kept under house arrest in the 1970s because of his independent views.

But the conservatives secured the election of Marxist economist Alexei Sergeyev, who has denounced the proposed transition to a free market as a betrayal of Communist ideals. Many of the new members of the Central Committee are little-known delegates nominated by their republics whose political views are difficult to discern.

The conservatives also pushed through an amendment to the party's new charter that will have the effect of making the party a more centralized organization than Gorbachev would have liked. The amendment restricts the theoretical independence of the Communist parties in each of the Soviet Union's republics by insisting that they adopt political programs and rules in line with those of the Soviet Communist Party.

Gorbachev and his supporters had argued against the amendment because they fear that it could hasten the break-up of the nationwide party as the republics continue to assert their independence. The Communist parties of the three Baltic republics have already split into pro- and anti-Moscow factions, and Communist party leaders in Georgia and Moldavia have threatened to break away unless they are given the right to develop their own political programs.