RUSSIA'S NIGHTMARE of an eruption of violence on its tinder- box southern flank became reality yesterday when at least 56 were killed, and more than 100 injured, in a city-centre bomb attack.

The explosion reduced to ruins a busy outdoor food market in Vladikavkaz, the capital of North Ossetia, a semi-autonomous republic within Russia, next to Chechnya.

Stunned, weeping crowds gathered in the city, a hotbed of ethnic tensions, after the blast which littered the area with body parts and the wreckage of stalls, buildings and cars.

As the dead were carried away by emergency workers using trolleys which usually carry fruit and vegetables, one witness compared the bloody scenes with "a meat grinder". The search for bodies buried under rubble continued into the evening.

The bomb exploded near stalls selling potatoes, and many of the dead and injured appeared to be pensioners and poor workers, witnesses said.

The bombing - the worst attack on civilians in the region since the Chechen war ended in 1996 - happened in mid-morning, when the market was packed with peasants hawking their wares, and shoppers with families. Local officials said the blast was caused by a home-made device, with the equivalent force of 10kg of TNT.

There were chaotic scenes in the notoriously ill-equipped local hospitals, which were overwhelmed by a flood of victims of shrapnel wounds.

The bomb is a further blow to the President, Boris Yeltsin, who is trying vainly to assert his authority after returning home from yet another stint in hospital. His credibility is already suffering badly from the fall- out over Russia's General Prosecutor - reinstated by regional leaders this week, despite being embroiled in a sex scandal - and a power struggle between the Kremlin and the legislature.

With news of the attack dominating the headlines, the President issued a statement condemning the bomb as a "barbaric" attempt at destabilising the north Caucasus", and dispatched his Interior Minister, Sergey Stepashin, and a security service investigation team to the scene.

It was Russia's second tragedy in 24 hours: only a few hours earlier, 23 people, many heavily sedated, died in a fire at a psychiatric hospital in Vologda, north of Moscow.

Last night, fears mounted that the attack could precipitate an outburst of prolonged bloodshed. Bombs, kidnappings and murders are common throughout the wild climes of the northern Caucasus, which has long been a seedbed for religious and ethnic conflicts, and mafia gangs. But the scale of the casualties in this case set it apart, lending weight to fears that Russia's part of the Caucasus is slipping beyond its control.

Ethnic tensions are especially complex in North Ossetia, where there are some 30,000 refugees from wars in nearby Georgia, Abkhazia, Chechnya and South Ossetia. But the search for an explanation will focus on the conflict between the Ossetians, who are Orthodox Christian, and the Muslim Ingush.

The two peoples have been at loggerheads since Stalin deported the Ingush to Central Asia in 1944, placing their lands under Ossetian jurisdiction.

In the immediate aftermath of the Soviet Union's collapse, tens of thousands of Ingush were driven from their homes in North Ossetia in a bout of ethnic cleansing which remains a source of profound bad blood. In 1992, several hundred people died in fighting between the two sides near Vladikavkaz.

Since then, violence has continued intermittently. If this is the cause behind the bombing, then more blood is certain to be spilt; the tradition of avenging blood with blood runs deep among the tribes of the Caucasus mountains.

However, there are other possibilities. Mafia groups abound in North Ossetia, not least because of its thriving bootleg vodka racket. One out of every six of the republic's 600,000 inhabitants is involved in cranking out moonshine for the rest of Russia.

Suspicions may also point at the lawless neighbouring republic of Chechnya, where relations with Moscow are at a post-war low after an audacious abduction in which a Russian Interior Ministry general was spirited off a plane at Grozny airport.