Terror in London: What we know so far about underground explosion

Here is what we know so far:

:: Emergency services including police, fire and ambulance were called to the scene at around 8.20am after an improvised explosive device detonated in a carriage.

:: The District line train, bound for central London, had just stopped at Parsons Green station in the west of the city when the explosion happened.

:: Twenty-two people have been injured, most of whom police said suffered "flash burns". London Ambulance said 19 patients, mostly with minor injuries, were taken to three London hospitals - St Mary's, St Thomas' and the Chelsea and Westminster. St Mary's earlier declared a major incident but has since been stood down. Three people "self-presented" with injuries, NHS England said.

:: Around two hours after the explosion, the Metropolitan Police confirmed they were treating it as a terrorist incident.

:: London Mayor Sadiq Khan told LBC radio "there is a manhunt under way as we speak". There have been no arrests.

:: US President Donald Trump denounced the attack by "sick and demented people who were in the sights of Scotland Yard". The Met declined to comment but Prime Minister Theresa May remarked: "I never think it's helpful for anybody to speculate on what is an ongoing investigation."

:: Witnesses said they saw a white "builder's bucket" in a supermarket bag, and pictures online appeared to show a flaming bucket with wires protruding from it inside a plastic carrier bag on the floor of a carriage. Reports suggested the device had a timer.

:: People reported seeing a "fireball" and a "wall of flame" in the train and people "covered in blood".

:: The public has been warned to expect an "enhanced police presence", particularly across the transport system, on Friday.