Canada's economic engine and most populous province of Ontario on Wednesday was ordered into a four-week lockdown to slow the spread of a new wave of Covid-19 infections.

The forcing of residents to stay home and the closing of retail stores except for curbside pickup comes just days after already-hardened public health restrictions went into effect, but have now been deemed insufficient to contain the outbreak.

"The Covid-19 situation is at a critical stage and we must act quickly and decisively to stay ahead of these deadly new variants," Ontario Premier Doug Ford told a news conference. "This will be in fact for four weeks."

This marks the second time so far this year that a province-wide emergency has been declared.

Ontario, which has been the worst hit by the pandemic in Canada, added 3,200 new cases or almost three-quarters of all new Covid-19 infections across the country in the past day.

More transmissible virus variants, which account for two-thirds of new cases in the province, are outpacing a stepped-up vaccination rollout.

Under pressure from doctors and nurses worried that hospitals would soon be overrun, Ford last Thursday pulled "the emergency brake for the entire province," imposing severe restrictions on services in the province of 14 million people, but stopped short of a complete lockdown.

People still went out en masse over the Easter long weekend as the weather warmed up, earning a rebuke from the premier.

"A lot of people were going into the malls and doing their little wander around and coming out with no bags," he said on Tuesday. "I'm sorry, but going to the malls is not essential."

The number of patients being treated for Covid-19 sickness in Ontario's intensive care units since then has hit a record 500. And public health officials project this and the number of Covid-19 cases could double within weeks.

The new lockdown is to take effect at 12:01 am (0401 GMT) Thursday and last until the end of April.

Only grocery stores and pharmacies will be permitted to stay open for customers to shop indoors.

Schools have been excluded from the lockdown, but some regions such as Toronto and surrounding communities have already temporarily halted in-person classes.

Meanwhile, the province's vaccines program, officials said, is to start targeting about 90 "hot spot" communities disproportionately impacted by outbreaks.