Police have increased patrols at a bayside beach after a group of youths allegedly stole mobile phones and purses from the bags of swimmers before smashing a bottle over the head of a 19-year-old man.

Key points: A 19-year-old man sitting on the beach was hit over the head with a glass bottle, police said

A 19-year-old man sitting on the beach was hit over the head with a glass bottle, police said Mobile phones and purses were stolen from the unattended bags of swimmers

Mobile phones and purses were stolen from the unattended bags of swimmers Police were called to the beach hours earlier due to an altercation involving a jetskiier

Extra officers were deployed to Chelsea beach in Melbourne's south-east on Friday night after a group of youths allegedly assaulted five men, one with a glass bottle, and others with a yellow plastic cricket bat about 9:30pm on Thursday.

Police said the group — whose members were described as being of African appearance and aged in their late teens and early 20s — "approached" the unattended belongings of swimmers on the beach.

Dozens of police officers patrolled Chelsea Beach last night. ( Supplied: Herald Sun )

A male swimmer who confronted them was assaulted by the group, which then fled with mobile phones and purses, police said.

It is alleged they attacked three other men aged 25 to 30 shortly afterwards, stealing the mobile phone of one of them.

Following this, a 19-year-old man who was sitting on the beach was struck in the head with a glass bottle, and was taken to hospital with non-life threatening injuries, police said.

Two others were treated by paramedics at the scene.

Police Minister Lisa Neville said Victoria's beaches were safe. ( ABC News: James Oaten )

Nobody has been arrested and police are keen to talk to witnesses.

In the wake of the incident, Superintendent Sharon McKinnon said she was confident the force had the right plans and resources to keep the city's New Year's Eve celebrations safe, and said police numbers on Chelsea beach would be immediately increased.

"I put my hand on my heart, that we will be working very hard with local council and using all our police to allow them [local residents] to enjoy that environment."

'To see this occur really has disturbed us deeply'

Superintendent McKinnon said the attacks were unprovoked.

"We are so disturbed by it," Superintendent McKinnon said.

"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.

"To see this occur really has disturbed us deeply.

"We do not want people not going to the beach — it's one of the most important parts of our wonderful culture that we have in this country.

"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."

She also said it was not unusual for there to be incidents on hot nights on the city's beaches, especially involving young people.

Police said they did not want the attacks to deter people from going to the beach. ( Supplied: Herald Sun )

Earlier on Thursday evening, police were called to the beach to deal with an altercation between two groups over a jet ski near the pier.

A 20-year-old man was arrested for being drunk and criminal damage, police said.

"They were separate groups, and we increased our numbers of police at the time, and we were able to calm that group down," she said.

"Everything seemed to be quite peaceful on the beach until these [later] incidents occurred."

Victoria Police said it would deploy extra resources wherever they were needed. ( ABC News: James Oaten )

Chelsea resident Roula Krikellis said she arrived at the beach around 7:30pm to find 30 police officers patrolling the area following the altercation over the jetski.

"I thought 'ok, something's going on here' … I then had a sense of uneasiness, this didn't seem normal at the beach," Ms Krikellis said.

She commended police on the "calm" way they handled the incidents, which she said would not deter her from enjoying her local beach.

"At the end of the day, there's instances [of crime] all over Melbourne, Australia, all around the world — we can't stop going to our beautiful beaches," she said.

The nearby St Kilda foreshore has been the site of several violent incidents this year. ( AAP: David Crosling )

New security cameras installed at St Kilda foreshore

The unrest came as the Victorian Government announced 10 new CCTV cameras had begun operation on the St Kilda foreshore, after revellers last year trashed the beach on Christmas Day.

Police Minister Lisa Neville said the high-profile law and order operation at St Kilda had not simply pushed the problem to other bayside beaches such as Chelsea beach.

"We had a small group, last night, who attempted to create harm," Ms Neville said.

"But my message is our beaches are safe, right across Victoria's bays and we have police at each and every [beach]."

Victoria Police said it was readily able to deploy and bolster forces wherever needed.

"We have a lot of extra police in the area, over and above what is generally here," Superintendent Tony Silva said.

"We'll alter our plans. We'll do that daily. We'll do that during the day if we need to do that. So if the intelligence says we need to beef up resources at Chelsea, we need to extend our time down there, and we need to work with council better, we'll do that."