Chechen rebels, striking with stealth and speed, fiercely assaulted Russian forces in at least three towns today, keeping their promise to harass Moscow's troops. The surprise attacks killed 26 soldiers, the biggest official one-day Russian death toll of the war.

Rebels entered the towns of Shali, southeast of the besieged capital of Grozny, and Argun and Gudermes, located to the east. All three had been under Russian control for six weeks or more, but the attack on Gudermes, Chechnya's second largest city, was especially startling. The city was regarded as so secure that Acting President Vladimir Putin used it as the setting for his surprise New Year's Day visit to front-line troops.

Russian officials said their troops and reinforcements beat back the rebels from each place, although the official Russian Tass news agency reported an attack after nightfall on the railroad station in Argun and continued shooting in Shali.

In Nazran, in neighboring Ingushetia, Chechens who communicate with rebels inside the war zone said fighting also took place in Urus-Martan and Achkhoi-Martan, both southwest of Grozny. Several Russian helicopters headed toward Urus-Martan in late afternoon, their rocket pods armed.

The rebel offensive coincided with the end of Ramadan, the month-long Muslim observance of fasting and prayer, and it appeared timed to take advantage of post-Ramadan euphoria among the rebels. It was also the second time in a week that the guerrillas suddenly appeared in towns under Russian control; last week, they briefly occupied the towns of Alkhan-Kala, Alkhan-Yurt and Yermolovsky, all of which sit just to Grozny's west.

These attacks--and an increasing toll of dead and wounded--have dimmed the aura of victory that surrounded the Russian ground offensive from its start more than three months ago. From a point around Christmas, when the fall of Grozny appeared imminent, to today, the Russian effort has fallen prey to inertia and guerrilla ambushes. A long-term, low-intensity war seems to have begun.

Russian officials resorted to misinformation in explaining the bad news from the front. Putin said the rebels took advantage of his decision to declare a weekend cease-fire during concurrent Orthodox Christmas and end-of-Ramadan festivities. But Putin actually only called a halt to air and artillery strikes over Grozny. Over the weekend, the Russians continued to fight on the outskirts of Grozny and pursue a mountain offensive in the south; attack helicopters were constantly at work over the capital and other towns.

"I took a decision to suspend all military operations. Subsequent developments showed yet again that . . . our enemies are people without clan or tribe or faith," Putin complained.

For his handling of the war, Putin's popularity has grown meteorically, first as prime minister and now as acting president. Polls released today indicated that he would win the scheduled March 26 presidential election without a runoff. In the face of bad news--and indications of battlefield mistakes and miscalculations--it remains to be seen whether the public will rally around him or abandon him.

Severe criticism of the war is beginning to emerge in the Russian press, which until now had been mostly docile.

"Everything is under control," a military analyst wrote sarcastically in Izvestia. Speaking of the towns hit by rebel attacks, he continued, "It suddenly turned out that none of these places had organized systems of defense."

The commentator, Yevgeny Krutikov, described a chronic Russian vulnerability to ambushes, the failure of much-touted loyalist Chechen forces to lend the Russians a hand and an uncoordinated offensive in the mountainous south that has left units encircled.

Today's descriptions of fighting highlighted weaknesses in the way the Russians array their troops against a mobile enemy. Reports said that the Russians were under attack inside various administration buildings in the three towns. Their vulnerability attests to the fact that in urban areas, troops are usually holed up in bunkered buildings, while the streets are free for almost anyone to roam, especially at night.

Ambushes of reinforcement units suggested that the guerrillas also can move without hindrance in the countryside. Russian checkpoints are limited to roads, troops do not patrol on foot, and most soldiers stay in base camps surrounded by trenches. There is plenty of space between the camps for guerrillas to penetrate populated areas.

Dmitri, a young armored vehicle commander hospitalized for facial burns, told reporters how his unit was ambushed on the way to the rescue of a convoy under fire. His short tale was full of signs of disorganization and badly thought-out military moves. Five armored personnel carriers were supposed to go on the mission, but only three actually went. No one scouted the route ahead. The result: Four of 24 soldiers were killed and eight were wounded in the ambush. "We were not able to even approach our convoy," said Dmitri.

Rumors of a post-Ramadan offensive have been circulating here in Ingushetia for days. "Everybody knew about the possible counter-offensive . . . particularly [on] Argun and Gudermes, and they did nothing," wrote Izvestia. "It is obvious that the federal troops had a serious failure, in which all the miscalculations and mistakes were displayed at once."

Military officials altered the death toll throughout the day--from 26 to 6 to 16. But by day's end, a military press center in Dagestan, east of Chechnya, stuck with the number 26, the largest officially announced figure in the conflict so far. The Russians said anywhere from 80 to 150 Chechens died. Russia has never acknowledged a reported death toll of more than 100 of its troops in Grozny on Dec. 15 when an armored column was ambushed.

The Defense Ministry put a positive spin on the attacks: They signified Chechen desperation. "When a beast is driven into a corner, his agony starts," said Gen. Valery Manilov, deputy army chief of staff. North Caucasus commander Gen. Viktor Kazantsev hinted at personnel failures. He said a new "assault detachment" will be trained in a "quality way" to prevent such incidents.

CAPTION: Russian soldiers ride atop their armored personnel carrier near the village of Sleptsovskaya, in Ingushetia, after returning from fighting in Chechnya.