Police said Yury Kardanov, a Vladikavkaz resident, entered Kindergarten No. 4 at midday brandishing the two grenades, Interfax reported.
He talked to police and demanded a ransom, the report said. The amount was not immediately known.
But before the hostage-taker's deadline had expired, the hostage-taker blew up both grenades on the kindergarten's ground floor, where there were 5- and 6-year-old children, Interfax said.
Kardanov himself was hospitalized with grave injuries, the agency said.
Three children, two teachers and a police officer also were taken to the hospital with various injuries.
Vladikavkaz, about 1,500 kilometers south of Moscow, is the capital of the republic of North Ossetia, bordering the war-torn Chechen republic in the North Caucasus region.
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