Terrified commuters watched in horror as teenagers shot at each other across a busy train carriage.
Passengers sat in fear as a group of four youths took turns to blast at each other with a ball-bearing gun across a bank of seats.
Some were so scared they phoned police from the train to warn them what was going on.
Police were last night questioning three boys aged 13, 14 and 15 and a 20-year-old man over the shooting in Hove.
And they warned parents their children were being put in danger by being allowed to carry the dangerous weapons on the streets.
Officers were called to Hove railway station at 6pm on Monday after commuters on the 5.50pm train dialled 999 to say the gang had begun firing at each other while drinking cans of lager.
The youths carried on shooting the weapon at each other as they stepped on to the platform.
Moments later a woman called police to say the gang of male youths had stolen alcohol from Video Box in nearby Goldstone Villas.
One of the youngsters, she said, had a gun in his back pocket.
Officers searched the area around the shop and station and arrested four males on suspicion of possession of an imitation firearm and theft.
A weapon - believed to be a BB gun - was seized while the youths were taken in for questioning.
Soft airguns are classed as imitation firearms under the Firearms Act 1968, meaning anyone found in possession of a BB gun in public is likely to be arrested and fined.
Offenders could even be jailed for up to six months.
Police said the four youths were lucky they had not found themselves facing an armed response team and in danger of being shot by the force.
A Sussex Police spokeswoman said: "Members of the public were extremely perturbed by the actions of these youths waving the gun around in full view of everyone.
"They were taking potshots at each other on the train and scaring passengers."
The spokeswoman added: "People have died from BB gun pellets.
"We have had reports of people with these things where we have had to call out a firearms response unit.
"These kids could have found themselves face to face with a real gun.
"However, we didn't send armed officers on this occasion because the reports we were getting meant we were certain that it was an imitation firearm.
"But these weapons are manufactured in such a way even our trained police firearms officers cannot always tell them apart from the real thing."
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