Of the six new cases recorded in the 24 hours until midnight last night, none of them were in isolation throughout their infectious period.

However, they are all linked to previous cases.

In total, there were 29,361 tests received yesterday and 18,901 vaccines administered.

In Ballarat, there are feelings of despair throughout the city with reports the business bounce back from the previous lockdown had been slower than previous times.

Meanwhile, police have arrested 15 people after hundreds of protesters rallied in Melbourne's CBD against the lockdown.

Friday marks day one of the restrictions, with more than six million Victorians find themselves living under stay-at-home orders again, only 10 days after coming out of the last lockdown.

Victorians are under the same rules that applied during last month's lockdown, including the five reasons to leave home, the five-kilometre travel limit for exercise and shopping, and compulsory masks indoors and outdoors.

Protesters, some carrying placards and most not wearing masks, gathered at Flinders Street about 7pm on Thursday and moved into Swanston Street as police in masks gathered to try to disperse them.

Flares were lit and the crowd chanted "no more lockdowns," the Herald Sun reported while residents of nearby apartments shouted "Go home, idiots".

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Video on Twitter feeds showed police on foot and horseback fronting the crowd, and a line of officers shoulder to shoulder on the steps of Flinders Street Station.

One video showed two officers using handheld devices to spray protesters with what may have been pepper spray.

One protester shouted "COVID is fake" and another said he was angered by Victoria's sixth lockdown.

"We can't keep going through this, we are losing our livelihoods," the man from Melbourne's southeast told the Herald Sun.

Police said "hundreds of people engaged in an unlawful protest" and they made 15 arrests - nine people were held before their identity could be confirmed to issue fines and two were arrested for breaching bail and stating false names.

They said four people were expected to be charged on summons in relation to breaching their bail, activating a flare and hindering police.

Officers also issued fines to 16 others on Thursday night for breaching coronavirus restrictions including exceeding the public gathering limit, failing to wear a mask, being more than 5km from home and being away from home other than for a permitted purpose.

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"Four of the offenders who were arrested or fined are known to police as protest organisers," Victoria Police Assistant Commissioner Luke Cornelius said.

"They have been dealt with previously and it is expected will be presented to court on appropriate charges, such as incitement".

Police said an officer suffered minor cuts and abrasions after he was pushed to the ground and kicked by several offenders.

"Police will continue to review video footage to identify as many protesters as we can, in order to hold offenders to account, as has been the case for previous protests," Mr Cornelius said.

"Police are, together with the vast majority of Victorians who are doing the right thing, outraged that a small minority of people continue to engage in deliberate breaches of the CHO Directions, putting the lives and jobs of their fellow Victorians, as well as their police at risk."

The seven-day lockdown announced by Mr Andrews began at 8pm on Thursday after the state recorded eight new cases of the Delta variant of the coronavirus.

Mr Andrews said the decision to lock down was "incredibly painful" but there was no alternative and he was determined to avoid an extended lockdown..

"The alternative is not to be locked down for seven days, it's being locked down for seven weeks or more, locked down until we get to 80 per cent vaccination and that may not happen until Christmas time," he said.

"We have been through a three-month lockdown. That was 2020, we don't want that again."

Thousands of businesses have been forced to shut again, as the state government faces criticism that not enough warning was given before Thursday's announcement.

"The 8pm start, with three hours' notice, doesn't give any business enough time to plan," Victorian Chamber of Commerce and Industry CEO Paul Guerra said.

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