Scotland Yard has declared a terrorist incident after a blast sent a 'fireball' and a 'wall of flame' through a packed London Underground train.

%image(14761488, type="article-full", alt="A blast sent a "fireball" and a "wall of flame" through a packed London Underground train on Friday morning (Picture: Stefan Rousseau/PA Wire)")

London Ambulance Service said 18 people were taken to hospital following the explosion in west London during rush hour this morning but none are thought to be in a serious or life-threatening condition

Emergency services were called at approximately 8.20am to Parsons Green Underground Station 'following reports of a fire on the train'.

The Metropolitan Police said Deputy Assistant Commissioner Neil Basu, the national co-ordinator for counter-terrorism policing, 'has declared it a terrorist incident'.

It added: 'At present we are aware of a number of people who have suffered injuries.'

Prime Minister Theresa May is to chair a meeting of the Government's Cobra emergencies committee this afternoon to discuss the incident.

Commuters fled in terror after the blast and witnesses reported seeing several people hurt and 'covered in blood' after a 'flash and a bang' on the District Line Tube.

Emergency services including armed police rushed to the scene and cordoned off the station.

Pictures posted on social media appeared to show wires protruding from a flaming bucket inside a plastic carrier bag on the floor of a carriage.

The Met said: 'It is too early to confirm the cause of the fire, which will be subject to the investigation that is now under way by the Met's Counter Terrorism Command.'

Sylvain Pennec, a software developer from Southfields, near Wimbledon, was around 10 metres from the source of the explosion when fire filled the carriage.

'I heard a boom and when I looked there were flames all around,' he said.

'People started to run but we were lucky to be stopping at Parsons Green as the door started to open.'

He described the scene of panic as commuters struggled to escape the carriage, 'collapsing and pushing' each other.

Mr Pennec stayed behind to take a closer look at what he believed was the source of the explosion.

'It looked like a bucket of mayonnaise,' he said.

'I'm not sure if it was a chemical reaction or something else, but it looked home-made. I'm not an expert though.'

Chris Wildish, who was on the train, said he saw a 'device' in the last carriage.

'It was a white bucket, a builder's bucket,' he told BBC 5 live.

'Flames were still coming out of it when I saw it and had a lot of wires hanging out of it - I can only assume it was done on purpose.

'It was standing against the door of the rear-most carriage.'

Here is a timeline of attacks in recent years:

•June 19 2017: One man dies and several others are injured after a man allegedly rams his van into worshippers in north London.

Darren Osborne, 47, of no fixed address in Cardiff, is charged with murder and attempted murder after being accused of carrying out a premeditated attack on Muslims as they left a mosque on Seven Sisters Road, Finsbury Park.

•June 3 2017: Eight people are killed in a terror attack around London Bridge.

A van ploughed into people on the bridge before the three attackers carried out a knife rampage in Borough Market. The perpetrators - Khuram Butt, 27, Rachid Redouane, 30, and Youssef Zaghba, 22 - were shot dead by police.

•May 22 2017: Twenty-two people - including children - are killed in a bombing at a pop concert in Manchester.

Lone suicide attacker Salman Abedi detonated an explosive device as crowds of music fans, many of them youngsters, left Manchester Arena following a performance by US singer Ariana Grande.

•March 22 2017: Five people are killed in a car and knife attack in Westminster.

Khalid Masood drove a hire car over Westminster Bridge, near the Houses of Parliament, mounted the pavement and hit pedestrians before crashing into railings outside the Palace of Westminster.

He stabbed Pc Keith Palmer, 48, to death. Also killed in the atrocity were US tourist Kurt Cochran, Romanian tourist Andreea Cristea, 31, and Britons Aysha Frade, 44, and 75-year-old Leslie Rhodes. Masood was shot dead by police.

•June 16 2016: Labour MP Jo Cox is murdered outside her constituency office in Batley, West Yorkshire.

The mother-of-two, 41, was shot and stabbed multiple times by right-wing extremist Thomas Mair. He was later handed a whole-life prison sentence for her murder.

•May 22 2013: Fusilier Lee Rigby is murdered by Islamist extremists Michael Adebolajo and Michael Adebowale.

The 25-year-old serviceman was walking near his barracks in Woolwich, south-east London, when the pair rammed him with a car before attempting to hack off his head with knives. The killers were jailed at the Old Bailey in February 2014.

•July 7 2005: Four suicide bombers kill 52 and injure hundreds of others in blasts on the London Underground network and a bus.

Twenty-six died in the bombing at Russell Square on the Piccadilly line, six in the bombing at Edgware Road on the Circle line, seven in the bombing at Aldgate on the Circle line, and 13 in the bombing on a bus at Tavistock Square.