Canada's most populous province of Ontario on Thursday clamped down on big parties, unveiling steep fines for breaches of strict limits on the number of people attending social gatherings to try to prevent a second wave of Covid-19 outbreaks.

Organizers of private gatherings of more than 10 people inside homes and more than 25 outside will face a fine of up to Can$10,000 (US$7,600), while each partygoer could be fined Can$750, Ontario Premier Doug Ford told a news conference.

The fines -- coming after an uptick in new coronavirus cases as students return to classes -- are the harshest in Canada for breaking public health orders.

"This is a serious situation, folks. We will throw the book at you if you break the rules," Ford said, citing recent risky behaviors such as "drinking, hugging (and) kissing" at illegal parties hosted by "reckless, careless people."

"The second wave is coming," the Ontario Medical Association warned in a statement, calling for increased Covid-19 testing capacity.

The new restrictions will take effect in Ottawa and the Toronto region, starting on Friday.

They will not apply to public outdoor events or venues such as restaurants, cinemas or sports arenas.

Ontario also scaled back the allowable size of gatherings from 50 people indoors and from 100 outdoors, under the new regime.

Ontario and Quebec, the two provinces most affected by the pandemic, have each recorded around 300 new Covid-19 cases in recent days.

In total, Canada has recorded more than 140,000 coronavirus cases and 9,238 deaths to date.

Chief public health officer Theresa Tam linked in part the sudden rise in new cases to private parties where "physical distancing and wearing of masks were not followed."

This "increases the risk for COVID-19 spread getting out of hand," she warned.