Key points:

• Apex gang to blame for CBD violence, police say

• Four arrested, including one person for possessing a taser

• Resources boosted for taskforce investigating group

The group, known as Apex and based in Melbourne's southern suburbs, forced a shutdown of parts of the CBD, as police brought out capsicum spray and batons to break up the commotion.

Victoria Police Chief Commissioner Graham Ashton said Taskforce Tense was set up in November to monitor Apex and would now be given extra resources.

"The levels of violence exhibited by this group was an increase and escalation on violence ... that concerns us greatly and we condemn（谴责） it," Chief Commissioner Ashton said.

"The sort of behaviour [on Saturday night] will not be tolerated and we'll be cracking down （镇压；取缔） hard on this particular group we believe was responsible."

Terrified revellers at the annual Moomba festival were forced to take cover as gang members provoked police and bystanders （旁观者；看热闹的） on the corner of Swanston and Flinders streets, near Federation Square, just before 8:00pm.

Onlookers ran for safety and at one stage chairs were used as weapons by the brawlers.

Swanston Street was shut down and trams were stopped for about an hour. Ambulance Victoria said it treated a number of people after the incident.

One man was taken to The Alfred hospital with serious head injuries.

Police said four people were arrested, including one for possessing a taser, but others fled on foot.

A number of robberies and an assault were also reported in the area.

Witnesses said more than 100 people were involved in the fight.

Russ Mulry said several groups of people were chanting （念咒） at police.

"Chairs were picked up and chairs were thrown from these gangs of people — there was glass being thrown, it was scary," he said.

"It was disgusting; I am ashamed to be a Melburnian with that going on."

Police taskforce looking into gang activity

Commissioner Ashton said police believed Apex members may be behind a spate of recent car thefts and burglaries in Melbourne's south.

He said Apex members had also recently come into the city for White Night and New Year's Eve events "looking to cause trouble".

Commissioner Ashton said police would go through CCTV footage from last night to make further arrests.

Deputy Commissioner Andrew Crisp said he was pleased with how police responded.

"What we did see was a large group of mainly young men that came into the city that seemed intent on provoking （激怒） both members of the public and Victoria Police," he said.

"This type of behaviour, in any part of this state, is totally unacceptable, particularly in the city, particularly during Moomba, which is promoted as it is very much a family event."

'It was a chaotic scene'

VIDEO: Watch: People run from Federation Square during the brawl

Hon Weng Chong said he was getting ice cream with a friend at Federation Square when he saw youths "running around".

"They would go up to the top of the stairs and somehow there was something that sparked a stampede (惊跑；人群的蜂拥）," he said.

"They just came running all the way down, it was really crazy there was like so many of them running towards us.

"All in all it was a very chaotic scene."

Mr Chong posted footage on YouTube that showed one of the youths knocking down a fence.

He said it appeared the teens were organised.

"There were a lot of people at that time, and they were all quite organised in the way they were running," he said.

"It was a bit scary because you didn't know what they were going to do. It was disconcerting."

Roads Minister Luke Donnellan said the behaviour was very disappointing.

"[Saturday] night's riots were totally unacceptable, totally inappropriate behaviour," he said.

"Moomba is a family event and the last thing we want is a pack of — to put it mildly — dickheads, gangs behaving like that, and pretty much ruining the marvellous image that Victoria has."

Opposition police spokesman Edward O'Donohue said the fighting showed the State Government needed to fund police properly.

"It's time for Daniel Andrews and his Government to get their priorities right and give Victoria Police the resources they need," he said.

Lord Mayor Robert Doyle assured people they would be safe to attend the rest of Moomba, which continues on Monday.