BABI YAR, Soviet Union — Fifty years after the Nazis slaughtered tens of thousands of people, mostly Jews, in a ravine outside Kiev, a Ukrainian leader stood at Babi Yar and vowed that such an atrocity must never happen again.

"Anti-Semitism still finds in some places its speakers, but they will have to know they will get no support on Ukrainian lands," Leonid Kravchuk, the president of the Ukrainian Parliament, told a crowd of about 10,000 who gathered on a chilly autumn evening to honor those killed.

"The history of the relations between the Ukrainian and Jewish peoples . . . has both brilliant and black pages," Kravchuk continued. "But let us remember not just to preserve old wounds but to ensure this does not happen in the future."

Kravchuk's speech was the first time that the Ukrainian government has so publicly and unequivocally acknowledged before the world the anti-Semitism that led some Ukrainians to collaborate with the Nazis during World War II.

Soviet President Mikhail S. Gorbachev, in a message read to the memorial ceremony, strongly condemned the anti-Semitism that persists in his country today and criticized those government bodies and political groups that have failed to use perestroika (restructuring) to "create an atmosphere of rejection and condemnation of the manifestations of anti-Semitism."

"Babi Yar shows that Jews were among the first Nazi victims, both in our country and in all of Europe," Gorbachev declared. "The Nazis speculated on the lowest feelings of envy, national intolerance and hatred. They used anti-Semitism as a major means to infect people's minds with chauvinism and racism.

"Venomous sprouts of anti-Semitism sprang in the Soviet Union. The Stalin bureaucracy, which publicly dissociated itself from anti-Semitism, in fact used it as a means to isolate the country from the outside world and strengthen their dictatorial position with the help of chauvinism.

"The years of perestroika and renovation have radically changed the social atmosphere in the country," Gorbachev's message added. "However, manifestations of anti-Semitism that still exist in our everyday life play into the hands of some reactionary circles."

The statements were greeted with cheers from the crowd, who included survivors of Babi Yar, and their relatives occupying seats of honor in the outdoor amphitheater.