With more than 50 other world leaders looking on, President Clinton pointed his finger at Boris Yeltsin today and urged the Russian president to halt bombing and rocket attacks that have caused heavy civilian casualties in Chechnya.

Clinton's dramatic appeal, at a summit conference here of the Organization for Security and Cooperation in Europe, marked a sharp escalation of previously muted public warnings by his administration against Moscow's blunt-edged military tactics in the secessionist Russian region.

Yeltsin, in an address minutes before Clinton's, told the United States and its European allies to stay out of Russia's affairs, including the war in Chechnya, and suggested the U.S.-led bombing of Yugoslavia last spring deprived Clinton of the right to lecture Russia on what it does in Chechnya. Yeltsin then left the summit conference and returned to Moscow, cutting short a meeting with French President Jacques Chirac and German Chancellor Gerhard Schroeder.

But Russia's foreign minister, Igor Ivanov, stayed behind and negotiated a much-contested OSCE security agreement, to be signed Friday, that justifies intervention in another country's conflicts if regional stability is threatened. The agreement, which Russia had strongly resisted earlier, was accompanied by Russian authorization for the OSCE chairman, Foreign Minister Knut Vollebaek of Norway, to visit Chechnya to assess the situation there, Secretary of State Madeleine K. Albright told reporters.

Despite that demonstration of accord, the strong statements by Clinton and Yeltsin, played out in a formal summit session, emphasized the degree to which Clinton and several European leaders used the gathering to heighten pressure on Russia to de-escalate its war against Chechnya's Islamic rebels.

As Yeltsin and other leaders sparred here in Istanbul, Russian forces took the key Chechen town of Achkhoi-Martan today without firing a shot, repeating the peaceful occupation they organized last week in the republic's second-largest city, Gudermes. After weeks of bombing and staging just inside the Chechen border, the Russian troops have occupied wide swaths of Chechen territory in recent days, pressing from several directions toward Grozny, the capital.

In his remarks, Clinton sought to mix blame with praise, reminding Yeltsin of his heroism in 1991, when he defied an attempted coup d'etat, and suggesting his current actions are unworthy of that legacy.

"If the attacks on [Chechen] civilians continue, the extremism Russia is trying to combat will only intensify," Clinton said in his opening statement. "In order to isolate and undermine the terrorists, there must be a political dialogue and a political settlement."

Yeltsin, apparently anticipating such criticism, had tried to blunt it with his earlier opening statement in which he said Russia's attacks on Chechnya are an internal matter. The world community "needs a respectful dialogue, not mutual reproaches or sermonizing," he said. "You have no right to criticize Russia for Chechnya. . . . Suffice it to recall the aggression of NATO headed by the United States that was mounted against Yugoslavia."

The speeches, in the majestic Ciragan Palace on the Bosphorus, carried echoes of Cold War exchanges between Soviet and U.S. leaders. But the stakes clearly were different; Washington and Moscow are no longer enemies or champions of competing world views. Moreover, both Clinton and Yeltsin tempered their remarks with appeals for a harmonious Europe that would include Russia and several other former Soviet states.

In fact, Yeltsin greeted Clinton with a bear hug and invited him to Moscow after the speeches, when the two men met privately for an hour, White House officials said. National security adviser Samuel R. "Sandy" Berger said Clinton told him Yeltsin, who has suffered health problems, "had a firm handshake and was very vigorous. . . . He still has a mischievous sense of humor."

Yeltsin's spokesman, Dmitry Yakushkin, told Russian reporters that the president's accelerated departure was not intended as a display of pique, but rather had been scheduled.

But neither the Russian nor U.S. president backed away from his earlier comments. "He was very vigorous, and so was I," Clinton said after the meeting. Berger said "we will look at our schedule" for a possible Clinton visit to Moscow next year to discuss matters including nuclear and conventional arms treaties.

The challenge to Yeltsin occurred near the end of Clinton's opening statement, in which he took issue with Yeltsin's comments about Yugoslavia. Citing Serbian atrocities in Bosnia and Kosovo, Clinton said there will be no time "when we will ever be able to say we simply cannot criticize this or that or the other action because it happened within the territorial borders of a single nation."

Clinton then pointed to the Russian president and said, "One of the most thrilling experiences of my life" was in 1991 when Yeltsin--then the Russian president-elect at a key moment in the Soviet Union's breakup--stood on a tank in Moscow to face down anti-democratic forces who were trying "to take the freedom of the Russian people away."

"Your standing there on that tank said to those people, 'You can do this, but you'll have to kill me first,' " Clinton said.

Had Yeltsin been put in jail instead of the president's office, he said, "I would hope that every leader of every country around this table would have stood up for you and for freedom in Russia, and not said, 'Well, that is an internal Russian affair that we cannot be part of.' "

Yeltsin told his fellow leaders, "As a result of the bloody wave of terrorist acts that have swept over Moscow and other cities and towns of our country, 1,580 peaceful inhabitants of our country have suffered. . . . Nobody should be under any illusions on this score: There will be no negotiations with bandits and murderers."

Clinton later told reporters, "I would never criticize anyone taking vigorous action against terrorism." But Moscow's differences with Chechnya cannot "be solved exclusively by military strategy," he said, because it leads "to greater than necessary civilian casualties."

Russian forces moved into Chechnya in late September to put down separatist Islamic guerrillas that Moscow blames for cross-border incursions into the neighboring region of Ingushetia and for a series of bomb attacks on apartment buildings in Moscow and other Russian cities that killed almost 300 people. Russian forces have bombed Chechnya from the air and launched artillery and missile strikes against Chechen towns and villages.

Correspondent David Hoffman in Moscow contributed to this report.

CAPTION: President Clinton implied that Russian President Boris Yeltsin's actions in Chechnya are unworthy of a man who risked his life to prevent a coup in Moscow in 1991.