Parsons Green terror attack: Police launch manhunt after 'improvised explosive device' blast on Tube train

Counter-terrorism officers search for attacker after explosion on rush-hour train

Police officers and sniffer dogs walk near Parsons Green Tube station (Reuters)

Police have launched an "urgent" manhunt after that terror attack at London's Parsons Green underground station.

A major counter-terrorism operation is underway after at least 22 people were injured by an "improvised explosive device" detonated on a rush-hour tube train.

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The blast caused a stampede of panicked commuters at the west London station.

Scotland Yard said it was treating the explosion as a terror attack.

The Metropolitan Police's Assistant Commissioner, Mark Rowley, declined to say if any arrests had been made or provide details on the suspects. He said it was a "live investigation".

But London Mayor Sadiq Khan told the LBC radio station that a "manhunt is underway", although declined to give further details.

It is believed detectives could be helped "considerably" by the fact the device did not detonate completely and was not destroyed by the blast.

Pictures posted on social media appeared to show wires emerging from a flaming bucket filled with chemicals inside a Lidl supermarket freezer bag.

Pictures on social media appeared to show the improvised explosive device (AFP/Getty Images) (AFP)

Twenty-two people have been taken to hospital following the explosion, with most suffering from flash burns. None are thought to have suffered serious or life-threatening injuries.

Passengers reported seeing people with burns to their faces and bodies on board the Tube carriage.

The Met said its officers would have an increased presence on London's streets while the attack is investigated.

The force is reviewing CCTV footage and speaking to witnesses as its hunts those involved.

Mr Khan called for Londoner's to remain "calm and vigilant".

"Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life," he said.

Prime Minister Theresa May is also set to hold an emergency COBRA meeting in Whitehall on Friday afternoon to discuss the attack.