MOSCOW, JULY 12 -- Boris Yeltsin, president of the vast Russian republic, resigned from the Soviet Communist Party today, triggering a split in the once monolithic organization that has ruled the Soviet Union for more than seven decades.

Shortly after Yeltsin made a theatrical walkout from the landmark 28th Communist Party Congress, several leaders of the radical-reform faction called Democratic Platform announced that they, too, would leave the party. They said they intend to launch a rival left-wing party and to convene a fall congress of all "democratic forces" opposed to Communist Party rule.

The walkout by Yeltsin and the other radicals is likely to reshape the Soviet political scene, making possible the rise of an effective opposition to the ruling Communists for the first time in Soviet history. It came only hours after President Mikhail Gorbachev's most prominent hard-line rival, Yegor Ligachev, was defeated in a bid to win the post of deputy party leader.

Today's dramatic developments also overshadowed what is likely to be the biggest shakeup in the party's leadership since Gorbachev came to power in March 1985. Nearly half of the members of the party's ruling Politburo, including Ligachev, have been dropped from the list of candidates for the Central Committee, which sets party policy in the four-to-five year intervals between congresses.

The nominees to the new Central Committee had included Yeltsin, who was elected May 29 as president of the Russian republic against Gorbachev's wishes. But the 59-year-old former Moscow Communist Party chief strode to the rostrum to withdraw his candidacy on the grounds that he could no longer submit to party discipline.

"As the highest elected figure in the republic, I can only subordinate myself to the will of the people and its elected representatives. I therefore announce my resignation from the Communist Party of the Soviet Union," said Yeltsin to a chorus of whistles and cries of "Shame!" from conservative delegates.

A strapping figure with a shock of silver hair, Yeltsin then turned his back on his former colleagues in the leadership and marched briskly out of the hall and out of the party he had belonged to for 29 years. He walked past a group of waiting reporters in the marble-and-glass foyer of the Kremlin's Palace of Congresses and stepped into a waiting car without saying another word.

Gorbachev, who brought Yeltsin to Moscow from the Siberian city of Sverdlovsk soon after he became Soviet leader, showed no emotion as his former protege announced his break with the party. "That ends the process logically," he commented, referring to Yeltsin's 2 1/2-year transformation from Communist Party apparatchik to populist hero.

Yeltsin had said earlier that he would either resign from the party or suspend his membership so that he could maintain an impartial position as president of the Russian legislature. Yeltsin and Lithuanian President Vytautas Landsbergis are now the only non-Communists to lead any of the Soviet Union's 15 constituent republics.

Little more than an hour after Yeltsin's walkout, a leader of Democratic Platform declared from the floor that he and other prominent figures in the reformist faction also would resign, and he called for a division of the party's immense property holdings. The declaration was made by Vyacheslav Shostakovsky, rector of the Higher Party School in Moscow, which serves as a training ground for future party bureaucrats and members of the nomenklatura, or party elite.

"I and my colleagues came to this congress with the hope that it would lead to resolute change toward the democratic renovation of our party. Regrettably, our hopes did not materialize," Shostakovsky said.

Democratic Platform, which had campaigned for the transformation of the Communist Party into a Western-type social democratic party, has the backing of about 100 of the 4,700 delegates to the congress. But its leaders claim the support of up to 40 percent of the 19 million rank-and-file Communists across the Soviet Union.

The declaration was signed by most of the leading figures in Democratic Platform, including Leningrad Mayor Anatoly Sobchak, Leningrad deputy Yuri Boldirev and the group's Moscow organizer, Vladimir Lysenko. It was not immediately clear, however, to what extent it was supported by all members of the faction.

Another Democratic Platform member, Georgi Gusev, was loudly applauded when he accused the faction's leaders of ignoring the wishes of its majority. Other radicals appeared divided over what action to take, but many predicted mass resignations from the party.

Dozens of opposition political parties, ranging from monarchists to anarchists, have sprung up here since the Communists formally gave up their claim to a monopoly of political power last February. But none have been able to approach the Communist Party even remotely in its wealth, organization and pervasive presence at every workplace and government institution in the country.

The formation of a rival left-wing party led by many big-name politicians could help break the Communist Party's grip on political power in vast areas of the country. By announcing their intention to withdraw from the party as an organized group rather than individually, the dissidents were able to argue that they deserve a share of party property, including offices and printing plants.

The defection by the radicals followed a sharp defeat for the conservative faction in voting for deputy general secretary, the party's new number two post. The conservative standard-bearer, Ligachev, managed only 776 votes against 3,109 for Ukrainian party leader Vladimir Ivashko, who was Gorbachev's nominee.

Ligachev was not included on the list of Central Committee candidates nominated by party leaders from the 15 republics and the central leadership, meaning that he will have to give up his post as the party secretary overseeing agriculture. The 69-year-old hard-liner has frequently clashed with Gorbachev over agricultural policy, opposing moves to dismantle the grossly inefficient system of collective farms in favor of individual family farms.

Other Politburo members dropped from the Central Committee list include ideology chief Vadim Medvedev, who was criticized at the congress for a lack of imagination, defense-industry chief Lev Zaikov, former Russian president Vitaly Vorotnikov and economics expert Nikolai Slyunkov. Alexander Yakovlev, widely regarded as Gorbachev's closest ally in the leadership, will not serve on the new Central Committee because he wants to concentrate on state matters in the newly formed presidential advisory council.

The Central Committee candidates list includes historian Roy Medvedev, who was persecuted during the 1970s for his persistence in investigating Joseph Stalin's dictatorial rule and his criticisms of the leadership then headed by Leonid Brezhnev.

Ambassador to Washington Alexander Bessmertnykh is one of several top diplomats on the Central Committee list, while the Kremlin's leading Americanologist, Georgi Arbatov, was dropped. Election to the 398-member body will take place Friday, the last day of the congress session.

BORIS YELTSIN, rallying figure for Communist Party radicals, became on the of most popular politicians in Rusia by attacking the pace of Gorbachev's reforms. A flamboyant Siberian, Yeltsin defied Gorbachev's attempts to kick him onto the political sidelines by securing election May 29 as president of the powerful Russian Federation. Since then, the 59-year-old rebel has called for Russia to take control of its economy and loosen its ties with Soviet central authority. Yeltsin says he is quitting the party to devote full attention to the presidency.

DEMOCRATIC PLATFORM, a radical group, claims to represent up to 40 percent of the Party's total membership, but was represented at the 28th Communist Party Congress by only 100 of 4,700 delegates. Before the Congress began July 2, Democratic Platform members threatened to break away unles sweeping reforms were implemented. The group's Moscow organizer, Vladimir Lysenko, has long been pesimistic about the chances for real reform and has predicted that the part will eventually split into right, left and centrist factions, losing millions of members.

Compiled by James Schwartz