GROZNY, Russia (AP) _ Backhoes manned by Russian troops chewed deep trenches across the streets of Chechnya's capital on Wednesday as part of extensive preparations aimed at frustrating car bomb attacks.

Russian commanders suspect rebels may launch a wave of attacks on or about Sunday, which Chechens consider their independence day.

Although the ruined streets of Grozny are severely potholed, the backhoes have hacked at all but the main boulevards for two days to make them more impassable.

The military also has tightened security by building an extensive network of temporary checkpoints, so close together that none of the checkpoints are out of sight of others.

Rebels used trucks in a series of deadly vehicle bombings last month and truck traffic into Grozny now has been banned.

Sunday is the four-year anniversary of Chechen rebels retaking the city from Russian forces, the decisive move of the 1994-96 war that ended with Russian troops withdrawing.

In that battle, rebels stormed into Grozny using sewers and backstreets, surrounding about 5,000 Russian soldiers and leaving them without water.

Russian troops re-entered Chechnya last September, and rebels fled Grozny en masse in early February. But rebel bombings and hit-and-run attacks continue in the city, as well as elsewhere in the republic.

A fistfight erupted on a city street Wednesday between two factions of Chechens loyal to Moscow, all in uniform of a pro-Russian police force. The brawl ended with a shootout in which one man was shot dead.

Russian soldiers at a military checkpoint about 20 yards away watched _ but did not interfere _ in the dispute, which underscored the tensions within the Kremlin-backed Chechen administration.

This government is also targeted almost daily by rebel assassins.

On Wednesday a bomb ripped through the car of a bureaucrat in charge of agriculture in the Urus-Martan region about 12 miles southwest of Grozny.

The bomb detonated as Movsar Shamsayev stepped out of his car, killing him instantly, an official with the pro-Moscow administration, speaking on condition of anonymity, told The Associated Press.

In another sign of Russia's difficulty in establishing civilian rule, a Russian-sponsored scholarship program appeared to unravel this week as the roadblocks prevented students from attending exams.

Professors came to administer the tests, which will choose 320 Chechens to study at Russian universities, but found the military unwilling to open the roads.

``I should fulfill my duty to choose 320 students... but I don't know how to do it,'' said Movsur Ibragimov, a professor at Moscow's Pedagogical University.

The current war began after Chechnya-based rebels twice raided the neighboring republic of Dagestan a year ago. Russia also blames Chechen rebels for apartment bombings that killed about 300 people last year.

President Vladimir Putin, speaking Wednesday at a ceremony for Russian paratroopers killed in Chechnya, said that military campaign in Chechnya helped keep Russia from breaking up.

``Our decisive actions ... have prevented the process of the collapse of the state from beginning,'' Putin said.