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Islamic State supporters celebrating the Parsons Green Tube terror attack have chillingly bragged the 'bucket bomb' was a 'trial run' for a bigger attack that will "rock London".

MirrorOnline has seen messages on a secret chat group on Telegram - the social media messaging service popular with terrorists - being exchanged this morning between ISIS-supporting members in Syria, Iraq and throughout Europe.

In the messages they praise the attacker who left the bomb on the London Underground tube which injured at least 18 people - and declare that there is more to come.

They begin by exchanging messages like: "Allahuakbar! May Allah accept him. Ameen" - which means "Praise to God!"

They also mock Londoners before being "paranoid that even exploding balloons make them crazy and panic" - launching that "they think it's a bomb!"

Using icons that illustrate planes flying towards buildings and flames - and emojis crying with laughter - they exchange images of the 'bucket bomb' which security sources believe was intended to do much more damage than transpired.

(Image: Splash News) (Image: Twitter)

They are also sharing photographs of some of the people who were hurt in the attack - and their injuries - with such captions as: "Flash burns and fear."

Most chillingly, one of the members of the group claims: "If it exploded for sure it will rock London."

Another replies: "Next time Bro. This was a trial run."

The reply is "Ameen" - similar to the Christian end of prayer saying 'Amen'.

Another poster boasted: "If it exploded for sure it will rock London!

"What exploded was a partial explosion that means the device was faulty".

(Image: Splash News)

MirrorOnline is not publishing the name of the secret chat group which is deliberately hidden on the Telegram messaging service and cannot be searched for.

Membership is by invitation only - with 'someone having to vouch for you' - and the group has around 150 members.

Fears that this might have only been a trial run have been expressed by several people on Twitter today.

One user, Mike Dunphy, posted: "I fear this is just a trial run, and there is more to come #StandUpToTerror".

Another, Simon Hallows, posted: "Seems improbable that #parsonsgreen device is a one off. Why end of train not middle? Using a timer! Trial run? I hope I am wrong #stayalert".

And another Twitter user, Bodkin Point, posted: "Another UK terror attack. Light damage. Maybe a trial run?"

MirrorOnline reported earlier how multiple people were injured when the 'bucket bomb' exploded on a packed London Underground tube carriage during rush-hour - causing widespread panic.

It soon became clear that police were dealing with a 'terrorist incident'.

Witnesses reported a "fireball flew down a carriage" when a white 'builder's DIY type' tub or bucket in a Lidl bag 'with wires coming out of it' exploded filling a rear train with smoke at Parsons Green Station.

(Image: REUTERS)

(Image: Twitter) (Image: REX/Shutterstock)

Several passengers suffered burns to their faces and bodies among other injuries believed to have been suffered as panicked commuters rushed to get out of the packed train onto a busy platform.

There was a "stampede" to get out of the station into nearby Fulham Road causing a 'crush' with people worried there was a 'bomb' - screaming and shouting: "Terrorist!", "Attack!" and "Everyone run!".

(Image: PA) (Image: PA)

One passenger said that a "fireball flew down carriage and we just jumped out open door" while another said there had been a "wall of flame" coming down the train carriage "after a bang".

Another commuter said she "saw a bag, flash and a bang" - while another said he sensed a "chemical smell rather than burning smell" as the "flames went up rather than out".

One woman told the BBC she 'had to walk over other people who fell on the platform to get out of the Tube train'.

Security sources say that there was a timer attached to the device - suggested that the attacker may have left the train at one of the five stations on the District Line before Parsons Green from where it started its journey in Wimbledon.

Nobody has yet been arrested - as a huge manhunt is launched for a bombmaker on the loose in London.