"They were animals, just animals," the victim, known only as Andrew, said from his hospital bed on Tuesday.

Andrew, a 21-year-old landscaper, told Channel Ten he was king-hit, kicked and stomped on in the unprovoked gang attack.

He was working outside the St Kilda Life Saving Club when his colleague Nathan warned him that brawling teens were smashing their way through temporary fencing.

When the pair screamed at them to get off the work site, they were attacked. Alarms were triggered by the smashed fencing.

"They've all just stopped, looked at me, and without saying a word have come and attacked me," Andrew told Channel Ten.

Police have revealed they found numerous "items" including a knife at the scene of the brawl.

Police said a group of three youths were attacked by another group of 10 to 15 youths, leaving one 15-year-old male unconscious. He has recovered and has left hospital. According to witness Laura, who asked that her surname not be used, up to 50 people were involved in the brawl. She said she saw the "large" knife being pulled and also saw the 15-year-old being knocked out with a pole that had been pulled from the ground.

"A pole with a sign on the end of it," she said, "just belting the guy with it."

Police say the fight broke out on Jacka Boulevard in the seaside suburb about 7.30pm on Monday after a verbal altercation. Detective Acting Inspector Damien Jackson said it appeared to be a case of "mistaken identity."

Witness "Brad" told radio station 3AW that two groups of teenagers of African appearance were pushing and shoving each other on the beach before the fight erupted. However according to Inspector Jackson "there were multiple nationalities involved." Witness Laura said she saw African, Islander and caucasian youths.

The brawl then moved towards the Stokehouse restaurant, he said.

He said one of the youths appeared to be carrying a slingshot and was being held back by a female friend.

"All of a sudden, boom, a brawl took place and we bolted," he said. "It just ruined our night."

Brad said his family was enjoying the warm weather by visiting St Kilda beach for some fish and chips.

"To be be honest it's disappointing, I've got my kids asking 100 questions," he said.

"My nine-year-old said 'Dad, can you make sure all the doors are locked'. He's never asked that."

Police have appealed for the brawlers to hand themselves in and promised they would be caught. "I want to provide some reassurance to the community that what occurred last night was not acceptable," Inspector Jason Kelly said. "The matter will be fully investigated and those responsible will be identified.

"It's the start of summer and this sort of behaviour will not be tolerated." Detectives were examining CCTV footage, he said.

Anyone with information is asked to contact Crime Stoppers on 1800 333 000.

Know more? Contact the journalist: tom.cowie@fairfaxmedia.com.au