Russian President Vladimir Putin, who was in the city when the blast struck, visited the scene of the explosion late on Monday night and laid a bunch of red flowers at a makeshift shrine to the victims.

Witnesses said they saw passengers who were bloodied and burned spilling out of the train, whose door was buckled by the force of the explosion, and lying on a platform while smoke filled the station.

Russia has in the past experienced bomb attacks carried out by Islamist rebels from Russia’s North Caucasus region. The rebellion there has been largely crushed, but Russia’s military intervention in Syria has now made it a potential target for Islamic State attacks, security experts say.

There was no immediate claim of responsibility. Officials said they were treating the blast as an act of terrorism, but there was no official confirmation of any link to Islamist radicals.

Earlier, Russian media had broadcast closed circuit television footage of a bearded man they said was being sought by police as a suspect. But Interfax reported that the man had come forward and been eliminated from inquiries.

The news agency, quoting an unnamed law enforcement source, said that human remains examined at the scene suggested that the blast had been carried out by a suicide bomber. It said the police had identified a suspect with links to radical Islamist groups banned in Russia.

If it is confirmed that the bomb was carried out by radical Islamists, the Kremlin is likely to argue the attack underlines the importance of its campaign in Syria, where it is backing President Bashar al-Assad in a fight against Islamist militants.

But some sections of Russian society may see the metro bombing as proof that Putin’s decision to intervene in Syria has again made Russian civilians into targets.

Two years ago, the Islamic State group said it brought down a plane carrying Russian tourists home from a Red Sea resort. All 224 people on board the flight were killed.

Soon after the blast happened at 2.40pm on Monday, ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad station. One helicopter hovered overhead and then landed on a broad avenue to take away an injured passenger.

“I saw a lot of smoke, a crowd making its way to the escalators, people with blood and other people’s insides on their clothes, bloody faces,” St Petersburg resident Leonid Chaika, who said he was at the station where the blast happened, told Reuters by phone. “Many were crying.”

The National Anti-Terrorist Committee said an explosive device had been found at another station, hidden in a fire extinguisher, but had been defused.

Video from the scene showed injured people lying bleeding on a platform, some being treated by emergency services and fellow passengers. Others ran away from the platform amid clouds of smoke, some screaming or holding their hands to their faces.

A huge hole was blasted in the side of a carriage and the door blown off, with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage after the train had pulled into the station.

Russian TV said many had suffered lacerations from glass shards and metal, the force of the explosion amplified by the confines of the carriage and the tunnel.

The Australian government urged travellers in the city to avoid affected areas and the metro, but did not change it’s level of advice for the country.

There’s no indication any Australians have been injured or killed in the deadly subway bombing in the Russian city of St Petersburg, foreign affairs officials say.

“The Australian Embassy in Moscow is in contact with Russian authorities to determine whether any Australians were involved in the explosions on the St Petersburg metro on 3 April,” a Department of Foreign Affairs spokesperson told AAP.

“There are currently no indications that Australians were killed or injured in the incident.”

– Reuters