The 18-year-old, who suffered serious head injuries in the fall, underwent major surgery in the Royal Victoria Hospital where his condition has been described as "critical but stable".

Three youths, all aged 15, who were arrested in connection with the incident, remained in custody last night.

The youth, who is believed to be from the Lisburn area, was crossing the footbridge close to Belfast's Central Station with three friends at around 9.30pm on Saturday night when they were approached by four youths.

A source close to the investigation said the victim was attacked by the youths and, in a bid to escape, he jumped from the bridge which runs from Belfast's Lanyone Place to East Bridge Street.

"We don't know if he maybe didn't realise how high the bridge was or if he was so terrified about being attacked that he thought it was better to take his chances jumping off the bridge," the source said.

DUP MP Jeffrey Donaldson said the incident was "another example of the kind of thuggery that stalks the streets of Belfast and other towns of Northern Ireland."

He added: "It is tragic that this young man is fighting for his life. He was not bothering anyone. He was going about his own business and was an easy target for these thugs who have no place in society. I would urge anyone with information to contact police."

Belfast Lord Mayor Jim Rodgers said the spot where the teenager was injured has become a "no-go area" after dark.

"There should not be any no-go areas in Belfast but unfortunately there are and where this incident happened is actually one of those spots. A lot of people will not walk along there after dark," he said.

Mr Rodgers added: "People should feel safe walking through Belfast but they do not. This was an appalling incident and my heart goes out to this young man and his family. I really hope he pulls through and makes a full and speedy recovery."

Police last night said they are keen to hear from anyone who may have been in the area at the time of the incident.

A police spokeswoman added: "Three males, all aged 15, are now in custody and are helping police with their inquiries. The 18-year-old remains in hospital where his condition is described as critical but stable.

"Police are re-issuing their appeal for information. Anyone who was in the area at the time and witnessed the incident is asked to contact detectives at Musgrave Street station on 0845 600 8000, or information about crime can be passed anonymously to the Crimestoppers charity on 0800 555 111. "