Young teenagers at a birthday gathering on Hampstead Heath were stabbed and bottled by a gang of older youths who robbed them of mobile phones and cash.

The revellers from schools in the area including Highgate, UCS, King Alfred, Marylebone, and Channing had congregated at the top of Parliament Hill on Friday evening after the event was advertised on Facebook to celebrate a girl’s 15th birthday.

“There were about 60 of them there and as they chatted and socialised it appears that some older members of a local gang began mingling in. Suddenly they put on balaclavas and pulled out weapons. They pinned down some of the boys and demanded they hand over their phones.” said the mother of one victim, who asked not to be named.

Witnesses said that one boy was stabbed and another was bottled in the head. Both victims needed hospital treatment.

One teenage girl, whose friends were at the event, said: “It was really scary. Everyone was just having a good time when suddenly everything changed. These boys had been chatting and blending in when they suddenly started attacking people. Girls were sobbing.”

Police were called to the scene at 9.50pm to reports of a man being stabbed near the hill. The police helicopter was scrambled and searched the area with ground officers for about an hour, but no victim could be found.

A Met Police spokesman said there were “no subsequent reports of anyone suffering injuries which could be linked to this location”.

But while officers were there they did arrest a 15-year-old boy on suspicion of going equipped for burglary. He was detained for questioning.