Russia's acting President Vladimir Putin today replaced a top commander in Chechnya whose troops have been criticized for doing a poor job of preventing Chechen guerrillas from infiltrating Russian-occupied towns.

Col. Gen. Vyacheslav Ovchinnikov was the third commander to lose his job since Jan. 1, the day after Putin succeeded former president Boris Yeltsin. Ovchinnikov was replaced by Col. Gen. Vyacheslav Tikhomirov, a Defense Ministry official.

The other ousted commanders, army generals Vladimir Shamanov and Gennady Troshev, were replaced by deputies and given jobs outside the theater of battle.

Ovchinnikov's troops, under the Interior Ministry, had been criticized for incomplete searches for separatist rebels in towns under Russian control. Their presumed laxity is thought to have contributed to the ease with which the Chechen guerrillas launched several surprise counterattacks in supposedly pacified areas, Russian officials said.

Moreover, guerrillas ambushed a top Russian general last Tuesday in a part of Grozny where Interior Ministry troops were responsible for security. The general's fate is unknown; the rebels said he is captive, but have yet to produce evidence. Russian military sources said he was killed by a sniper.

Hit-and-run rebel tactics this week have slowed Russia's effort to storm Grozny, the bombed-out regional capital, and made areas behind Russian lines insecure. According to Russian officials, Grozny was supposed to have fallen before today. On Tuesday, when Russian forces broke through to central neighborhoods, officials said they needed only three or four days to clear out the rebels.

Reports from Grozny remained inconclusive today. The army was advancing, but the rebels put up stiff resistance, military officials said. Fierce street fighting was reported and heavy rocket fire rained on buildings from which rebel snipers pinned down Russian troops.

The Russians claimed again that they had retaken control of a bridge over the Sunzha River, a major transit route for the rebels, and of central Minutka Square. Each has been reported captured at least three times this week.

Russian television showed footage of troops raising a flag over a building in the Sixth Microdistrict, in far eastern Grozny, representing an advance of about 400 yards from Russian positions in Staraya Sunzha.

From a border vantage point near the main highway that runs east to Grozny from Ingushetia, rocket-laden trucks could be seen rushing toward the city. Soldiers sitting listlessly atop nearby armored transport vehicles said weeks, rather than days, might be needed to rout the rebels. Refugees from Grozny carried tales of guerrilla snipers popping out of basements and sewers to shoot at Russian positions.

On Friday, Ovchinnikov, the ousted general, gave a downbeat assessment of the war. He advised that even when Grozny falls, occupation forces will come under threat. "I fully realize the difficulties these troops will encounter. The main threats will come from international terrorism," he said. "Mercenaries will try to cross the border and carry out subversive operations against army detachments and units of the internal troops."

Residents continued to flee the fighting. Zainad Abdul-Lapagova, a mother of two, left Grozny on Friday and was waiting to cross into Ingushetia today. She said her Staropromyslovsky neighborhood was a shooting gallery. "On our street there were six tanks parked right next to houses," she said. "Maybe 300 meters away, the guerrillas were sniping. The tanks fired and our house shook constantly."

But it was not the four months of fighting that drove Abdul-Lapagova and her sister from home. It was realization that at any moment, she could be in the middle of a battle. "I went to a well for water. This is always dangerous, with the snipers around," she said. "Suddenly, a fighter appeared from out of a manhole. I said nothing. He said nothing. He just sunk into the ground again. . . . There was no way to know where the shooting would start."

Despite the dangers, she said about 40 civilians, mostly elderly, remain huddled in a basement on her street. "People who are left are incapable of getting out--they are trapped," she said.