Police have been told some swimmers were in the water when the group started rummaging through their unattended belongings on the beach. A male swimmer spotted the thieves and confronted the group and was assaulted before the group fled with mobile phones and purses. A short time later, police said the group approached three men and assaulted them. One of the victims, a 28-year-old man, also had his phone stolen before the young males fled down the beach. The group then approached a 19-year-old man who was sitting on the beach and glassed him in the head with a beer bottle.

He was taken to hospital where he received stitches. Two other victims were also treated by paramedics for minor injuries at the scene. The offenders are described as being of African appearance and aged in their late teens or early 20s. Police say they will be back on Chelsea beach on Friday night. One Chelsea local, who spoke to The Age on the condition of anonymity, said he was standing on the edge of his balcony overlooking the beach when he saw two groups of young men in a stand off at about 6.30pm.

‘‘They started screaming at each other and within a few seconds they started punching and brawling,’’ he said. ‘‘About a dozen police were there within seconds trying to break up the two groups and then shortly after that another 20 police officers stormed the area.’’ The resident said earlier that evening he had counted about 25 jetskis within a 100 metre stretch of water. ‘‘I saw some of the men driving recklessly on the jetskis ... against water laws ... they were going right up to swimmers in the water and just causing trouble,’’ he said. The local, whose family has lived in the area for more than 40 years, said he no longer felt safe to go to the beach on hot nights.

‘‘It’s a real problem in Chelsea, the area has really deteriorated in recent years,’’ he said. Loading ‘‘There are gangs of youths, aged in their teens or early 20s, they hang out at the beach all night. A lot of the time they look like they’re on drugs or they’ve been drinking even though there are alcohol bans in place along the foreshore. "Sometimes you can smell the marijuana wafting from the beach and you often hear yelling and fights. It’s really not good and you just don’t feel safe being out there.’’

The resident said the situation had been worsened by recent redevelopments in the nearby shopping strip. ‘‘There’s hundreds more car parks now so I reckon it’s brought carloads of people to the area,’’ he said. ‘‘They drink near the bottle shop then they all head down to the beach.’’ A caller to radio station 3AW said a teenager had tried to open his car's door as he waited at traffic lights in Chelsea on Thursday night but it was locked. He sped through a red light to escape but later found his vehicle's door lock was broken. While the attacks are part of an emerging issue, police say people should still feel confident about going to the beach over the weekend and into the new year with more officers set to be on patrol in the area. Superintendent Sharon McKinnon said police would be doubling police numbers for the Chelsea area on Friday night.

‘‘We are so disturbed by it, in fact those first three victims had been working hard all day in the heat have gone down in the heat in this lovely country of ours to enjoy the beach to see this occur has disturbed us deeply,’’ Superintendent McKinnon told reporters on Friday morning. ‘‘We will be working even harder to ensure it doesn’t [happen again] because we don’t want people not going to the beach ... it’s one of the most important parts of the wonderful culture we have in this country." "Going forward, we will be rethinking our Summer Safe program that we're running around how we better place our members in a preventative and proactive way to prevent something like this from ever happening again." Superintendent Sharon McKinnon talks to reporters on Friday morning. Credit:Eddie Jim Superintendent McKinnon also moved to reassure beachgoers that Chelsea would be safe on New Year's Eve.

Investigators are urging anyone with video footage of the incident to contact police. There are fears that the violence at Chelsea is a result of a police crackdown at St Kilda beach with offenders simply moving down the bay to their next coastal target. Earlier this month, shocking footage emerged of the moment two men were ambushed and bashed unconscious in an “unprovoked, random, senseless and brutal” attack by a group of 20 youths on the St Kilda foreshore. The victims were left unconscious, and one was robbed, after being repeatedly kicked and punched in the random assault on December 1. Loading Replay Replay video Play video Play video