A child was among three people killed today when a car exploded outside an underground station during the morning rush hour in St Petersburg.

Police said that the infant and two adults died when a hand grenade went off inside their vehicle near the Udeleyaya metro station in the north of the city.

Early reports suggested that the victims were a husband and wife who were travelling in an unlicensed taxi with their child aged two or three. The driver is said to be in a critical condition in hospital.

The Lada car was just 50 metres from the entrance to the station when the blast occurred at around 8.55am, the Investigative Committee of the St Petersburg Prosecutor's Office said.

"We can confirm that it was a grenade and its type will be determined by forensic experts," a spokesman said. The Ria-Novosti news agency reported that the authorities were ruling out terrorism and believed that the grenade belonged to one of the passengers.

An official with the Federal Security Service (FSB) told the Interfax agency that the grenade went off shortly after the driver stopped to pick up his passengers. The infant and man died at the scene, while the woman died shortly afterwards.

Officials said that the metro station had not been affected and services were running normally. It was not clear what or who the intended target of the grenade might have been.

Many cars double as unlicensed taxis in Russia, with drivers earning petrol money by carrying people who flag them down in the street. Attacks involving mafia groups and business vendettas are also common in Russia's main cities, where rival gangs wrestle for control of territory and lucrative local trading centres such as food markets.

In an unrelated incident, a senior judge in the southern Russian city of Samara was shot as she approached her car this morning. Lyubov Drozdova, chairwoman of the Samara District Court, was rushed to hospital with stomach wounds after being hit in a drive-by shooting.

Police said that the gunmen fled in a green Niva car. Detectives have begun an inquiry into attempted murder.

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