Melbourne beachgoers have been viciously attacked by a gang of African youths who allegedly smashed a glass bottle over a teenager's head before assaulting multiple swimmers and stealing their wallets.

The youths are accused of assaulting and robbing beachgoers in three separate incidents at Chelsea beach, south-east Melbourne, between 9.30 and 10pm on Thursday night.

Police had previously attended the beach to control two separate out-of-hand groups who engaged in a violent brawl at about 6pm.

Police were called to the beach at 6pm to control the crowd (pictured), hours before an African gang robbed and assaulted beachgoers in a separate incident between about 9.30 and 10pm

Chelsea Beach, south-east Melbourne, was infiltrated by a number of gangs who caused chaos on Thursday evening (pictured: two large groups get into brawl at about 6pm)

A number of swimmers were enjoying Melbourne's warm temperatures by taking a dip in the water when the group of African youths begun to rummage through their unattended belongings on the beach.

One male swimmer was assaulted when he confronted the thieves who fled with mobile phones and purses.

The group later approached three other men aged 25, 28 and 30 who were also assaulted.

The 28-year-old's mobile phone was stolen during the altercation.

Moments after, a 19-year-old man who was sitting on the beach was approached by the group of thugs.

He was violently struck in the head with a glass bottle and assaulted with a plastic cricket bat, police said.

The man was taken to hospital with non-life threatening injuries but required stitches on his head.

Two other victims were treated on the scene by paramedics.

No arrests have been made and police have described all offenders to be African in appearance, aged in their late teens or early 20's.

A witness told The Age that he doesn't feel safe going down to the beach and that you could always smell drugs on the youths.

A witness said that he doesn't feel safe going down to the beach and that you could always smell drugs on the youths

'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,' he said.

'It's a real problem in Chelsea, the area has really deteriorated in recent years.'

Superintendent Sharon McKinnon spoke to the media on Friday morning and said police were disturbed by the incidents.

'Those three first victims had been working hard all day in the heat, and have gone down, as we all should, in this lovely country of ours, to enjoy the beach,' she said.

'To see this occur really has disturbed us deeply.

'We will ensure that we keep reviewing our work in this area, because it is an emerging issue, so that we can make our beaches safe.'

A public order response team was called in at about 6pm and a 20-year-old man was arrested for being drunk and for criminal damage

She promised residents that there is nothing for them to worry about and that there would be more police sent out to keep the area safe.

Earlier in the evening police were called to the beach over an altercation about a jet ski near the pier just after 6pm - in a separate incident.

The brawl erupted between two large groups, understood to be of Lebanese and Turkish heritage, causing fear among the local community, Nine News reported.

'Uneasy, there was a little bit of fear because your mind went everywhere,' said resident Roula Krikellis.

Footage captured from the rampage show a number of young men running at each other in an aggressive tussle.

A public order response team was called in at about 6pm and a 20-year-old man was arrested for being drunk and for criminal damage.

Ms McKinnon said police were able to control the crowd - until a string of assaults and robberies were carried out later in the evening by a different group.

'We have increased our numbers of police at the time and we were able to calm that group down and everything essentially seemed to be quite peaceful one the beach,' Ms McKinnon said.