President Boris Yeltsin, saying that "terrorism has declared war on us," ordered increased security at nuclear power plants and other sensitive installations around the country following the third apartment building bombing in Russia in nine days.

At least 103 people, including six children, were killed in this morning's blast at a building south of Moscow, and dozens more were reported missing, officials said. Yeltsin did not accuse any group of planting the bomb, but officials said suspicion has fallen on guerrillas from Chechnya who are battling government forces in the Caucasus region of southern Russia.

Yeltsin vowed to take "tough, swift and decisive" action but stopped short of imposing a nationwide state of emergency. He also ordered tightened security at airports, pipelines and train stations and gave Moscow police authority to remove anyone from the city who is not officially registered to live here.

Mayor Yuri Luzhkov said that Moscow, home to more than 8 million people, is visited by nearly 3 million people daily. He ordered that all visitors re-register with police within three days, but officials noted that many thousands are undocumented workers and migrants who never registered in the first place. Authorities acknowledged that they are targeting visitors and cargo from Chechnya.

Moscow city officials imposed new security restrictions on markets and highways here and began the arduous task of checking 35,000 apartment buildings for bombs. But Yeltsin, Luzhkov and other leaders said they do not want to declare a formal state of emergency; politicians almost universally expressed concern that such a declaration could be used as a pretext to abort parliamentary elections scheduled for December.

The blast at 6 Kashirskoye Highway struck with deadly efficiency at 5 a.m., leveling the eight-floor brick building. Records show that 142 people lived in the building but that about 30 were not home at the time of the blast. The blast site is about four miles from that of an apartment building explosion that killed 94 people last week, and today's blast came on a national day of mourning for victims of that explosion and one nine days ago.

Federal Security Service spokesman Alexander Zadonovich said the "handwriting is the same" in the two Moscow explosions. "The criminals are applying the following scheme, as we have established now: They rent space--either a [storeroom] or an empty space--for some period of time; they pile up explosives, and then there is a blast," he said. He added that today's explosion occurred in a space previously rented as a second-hand furniture shop. Police said they believe the explosive substance used in both Moscow bombings was hexogen, also known as RDX, a military explosive that is sometimes used in the oil industry.

Police said also that the bombers apparently used the identity of a man, Mukhit Laipanov, who died several years ago to lease space in both Moscow buildings. Authorities were circulating a photo of a man suspected of playing a role in the bombings.

In the earliest of the recent bombings, on Sept. 4, a car bomb detonated outside an apartment building for Russian military families in the troubled region of Dagestan, killing 64 people. Dagestan borders Chechnya, and guerrillas there who fought an inconclusive war for Chechen independence several years ago have carried their fight to Dagestan in recent weeks.

In the predawn darkness here this morning, rescuers raced to find survivors of the latest blast, but there were few. Most of the injured were residents of neighboring buildings who were hit by flying debris. Rain hampered efforts to search for anyone who might have been trapped under tons of smoldering rubble.

In a televised address, Yeltsin described the attack as terrorism and added: "This enemy does not have a conscience, shows no sorrow and is without honor. It has no face, nationality or belief. Let me stress--no nationality, no belief."

Earlier, Yeltsin assembled top officials at the Kremlin, including Luzhkov and law enforcement chiefs, an action he did not take after last week's attack. He also summoned Prime Minister Vladimir Putin back to Moscow from an economic conference in New Zealand.

Secretary of Defense William S. Cohen, who is in Moscow for meetings with his Russian counterpart, Igor Sergeyev, called the bombing "a cowardly and callous act of terrorism" and reiterated an earlier pledge by President Clinton to cooperate with Russia against terrorism.

Chechen guerrillas crossed into Dagestan two weeks ago and seized control of at least six border villages and a town, saying their aim is to establish an Islamic state in the region. Just days earlier, Russian troops had expelled a second, separate group of guerrillas from two villages in central Dagestan who had declared Islamic rule there. The latest fighting between the guerrillas and Russian troops has been intense, with the Russian military using warplanes, tanks and heavy artillery to try to dislodge the Chechens.

Vyacheslav Izmailov, a journalist who for four years has been helping gain the release of kidnap victims in Chechnya, said in television interviews that the guerrillas had formulated a strategy to strike back at Russia through the bombings. Izmailov said he had reported to authorities that the Chechens had sent "groups of terrorists" to Russian cities--using Slavic agents rather than ethnic Chechens who might be more conspicuous--to plant bombs. The bombers, he said, were to receive $50,000 for each successful blast.

Izmailov wrote in the newspaper Novaya Gazeta today that 10-man teams of bombers drove into Russian cities without hindrance. His claims were not confirmed by authorities, but one law enforcement source said his information is being taken seriously. In Grozny, the Chechen capital, the government denied any responsibility for the blasts.

Yeltsin's decision for now not to resort to declaring a national emergency was widely hailed. Russia still lacks a contemporary law on procedures for declaring a state of emergency, the last having been enacted in 1991, before the current Constitution. Parliament is scheduled to discuss the issue Tuesday.

CAPTION: Russian authorities are distributing this artist's impression of a suspect in the two Moscow bombings.