MOSCOW -- The anti-Communist opposition movement Democratic Russia denounced yesterday an explosion that wrecked its headquarters Thursday night as "political terrorism," but vowed that it would not derail the campaign of Boris N. Yeltsin for the Russian presidency.
"It would be tragic if this started a chain of terrorist acts in Moscow," said Vladimir Bokser, a Democratic Russia leader.
The blast may have destroyed 200,000 signatures supporting Mr. Yeltsin's campaign, but far more than the 100,000 necessary for registration were preserved in other locations, Mr. Bokser said. In Mr. Yeltsin's hometown of Sverdlovsk alone, well over 100,000 signatures have been collected, Sverdlovsk journalists reported.
A poll in 14 cities, reported on Russian television, made Mr. Yeltsin look virtually unbeatable in the June 12 contest, the first contested, democratic election of a Russian leader in history.
He was backed by 52 percent of voters polled, with former Soviet Prime Minister Nikolai I. Ryzhkov, who has been endorsed by the Russian Communist Party, a distant second at 10 percent. Vadim V. Bakatin, an adviser to President Mikhail S. Gorbachev, got 2 percent.
Speaking at a press conference, Mr. Bokser said that a natural gas leak had been ruled out and that bomb experts estimated that it was caused by 5 to 11 pounds of high-powered explosives. Pieces that could have been from a timing mechanism had been found, he said.
The explosion leveled the central portion of a century-old brick building Democratic Russia shared with a scientific institute on a back street across the Moscow River from the Kremlin. The one Democratic Russia activist in the building was in a back corner and was not injured.
Mr. Bokser and other Democratic Russia leaders said they did not know who was responsible for the attack. Mr. Bokser said it was possible to say for certain only that the terrorists were "political opponents of the democratic movement" and "extremist or fascist," because of their disregard for human life.
Late Thursday night, some activists at the scene of the blast speculated that the Communist Party, possibly with the aid of the KGB, was behind the bombing. That speculation was widely shared by Muscovites on the street yesterday who signed petitions backing Mr. Yeltsin or supporting Gavriil K. Popov, another Democratic Russia candidate, for mayor of Moscow.
"Democratic, Be Vigilant -- A Doomed Regime and a Mad Dog Are Twin Brothers," said a poster held by Anatoly Cherapanov across from the Moscow City Council.
"I don't know whose hands did it [the explosion] -- MVD [Ministry of Internal Affairs], KGB, or criminals for hire," Mr. Cherapanov said. "But I know it was ordered by the criminal Bolshevik regime."
People crowding around spoke out in agreement.
But the Democratic Russia leaders were wary of accusing the KGB or Communist Party.
"I don't think it was the KGB," said Lev A. Ponomaryov, a member of the Russian parliament. "But I'm absolutely certain that if it wants to, the KGB can prevent any more explosions from taking place. Its job is to defend democratic institutions."
Father Gleb P. Yakunin, a radical Orthodox priest and former political prisoner, sort of agreed. "I don't think [KGB Chairman Vladimir A.] Kryuchkov or another high official gave the order," he said. "But it's entirely possible that some officers of the KGB are ready to do anything to go back to old times."
The explosion took place a few hours after the U.S.S.R. Supreme Soviet passed the first comprehensive law governing the conduct of the KGB. Mr. Kryuchkov hailed it as providing a legal basis for the operations of the security police.