MOSCOW, JULY 13 -- The mayors of Moscow and Leningrad -- the Soviet Union's two largest cities -- said today they are leaving the Communist Party, following Russian republic President Boris Yeltsin in a growing exodus that could ultimately transform Soviet politics.

Moscow's Gavril Popov and Leningrad's Anatoly Sobchak, both radicals who were elected mayor in recent months, said in a joint statement that they will resign from the party to work for a multi-party system in the Soviet Union. They said the 28th Party Congress, which ended tonight in Moscow, had shown its "complete inability to offer the country a real program of transition to a new society."

In a separate press conference this afternoon, five other departing party leaders outlined their plans to form a new democratic party, based on a parliamentary model, and called on other disgruntled Soviet Communists to join them. All are members of the Democratic Platform, a radical faction of the party that has called an emergency meeting this weekend to discuss the future shape of Soviet politics.

By the end of the day, the Democratic Platform itself was split, with about half the 100 delegates who belong to the group saying that they would prefer to stay in the party and work for change from within.

The meeting of nearly 4,700 delegates, which convened last week, ended with a rousing speech from party leader Mikhail Gorbachev and the election of a new party leadership.

Interviewed on the floor of the massive Kremlin building where the congress is being held, many delegates expressed anger about the resignations. Some denounced the departing members as "extremists." Others proposed that they be stripped of their academic honors and official awards.

During a break in the day's session, Gorbachev, too, criticized the defections. "Those who leave now and seek refuge elsewhere, I view this with contempt," he told CBS television in a brief interview. "There's no question, I'm not veering from my course, and I have many supporters."

A day after his dramatic walkout from the congress and the Soviet party, Yeltsin convened a meeting of the parliament of the Russian republic, but he declined to discuss his resignation or future plans. He told deputies that his decision was "made with a heavy heart" but otherwise asked them not to discuss it.

In an interview with the Moscow Radio news service Interfax, however, Yeltsin said he was "dissatisfied" with the congress and condemned the Soviet Communist Party as "doomed to lag behind." Soviet officials close to Yeltsin said that he is likely to remain unaffiliated with any party rather than help form a new political organization.

Still, Yeltsin's departure will lead to mass defections from the party, delegates to the congress and other Soviet political figures predicted.

"In leaving, Yeltsin has given many of those who are just fed up with the party just the push they were waiting for," said Alexei Vladimirov, a party delegate. "I have no doubts that the numbers who will follow him will eventually total in the millions."

Particularly in the Soviet ethnic republics and other areas of the provinces, where support for the party is weak, Yeltsin is expected to trigger an outflow of rank-and-file party members.

"Many of us view the conservative mood at the congress as proof that the party isn't prepared to reform," said Alexei Boiko, a deputy of the Supreme Soviet interviewed in his home district in the Ukrainian mining town of Donetsk. "And many us are ready to follow Yeltsin out the door of it."

Nevertheless, the core of the party represented at the congress, the most loyal and conservative groups, were not swayed by the departure of Yeltsin or other Communists.

Even some members of the radical Democratic Platform declared that they will stay in the party and issued a petition supporting it. "The battle to liberalize the party doesn't end with this congress," said Alexei Elyarevich, a Democratic Platform delegate from Leningrad. "Some of us are staying in the party to fight for liberalization from within." In all, it appeared that half of the approximately 100 delegates belonging to the Democratic Platform will stay in the party.

Today's press conference, called to announce plans for a new political party, was led by Democratic Platform members Vladimir Lysenko, Yuri Boldirev, Dmitri Smirnov, Stepan Sulakhin and Vyacheslav Shostakovsky.

They appealed to Communist Party members who are ready to resign to join them but asked that they not turn in their party cards yet. The organizers avoided characterizing the shape of the new party, except to say that it would be parliamentary in style and would not be Communist. The new party has scheduled a convention in Moscow in October.

Asked who would be its leader, they said that Yeltsin was the de facto leader of any democratic force in the Soviet Union.