"We are now fighting a new enemy", Ontario Premier Doug Ford said at a news conference Thursday afternoon.
The measures take effect Saturday and continue for at least four weeks.
The number of COVID patients in Ontario's intensive-care units (ICUs) is expected to reach 800 by the end of April, up from 430 now, already the most during the entire pandemic.
The third wave of the COVID-19 pandemic in Ontario is being driven by variants of concern and younger people are being hospitalized, with officials noting their cases are more acute and critical.
"While experts were warning him of explosive growth of more infectious and more deadly variants, he cancelled public health protections", said NDP Leader Andrea Horwath in a statement.
Schools and child-care centres are expected to stay open throughout the four-week shutdown.
Those figures follow weeks of rising case counts as more contagious variants of the virus spread across the province.
Indoor social gatherings are prohibited and outdoor gatherings will be limited to five people.
The rules are slightly more lax for religious, wedding and funeral services in Ontario, however, with indoor events permitted to host up to 15 per cent of a room's capacity.
Rather than introducing that measure, the province has encouraged people to get tested for COVID-19 so they can apply for federal benefits if the virus forces them to stay home.
Schools remain open because, the government says, they are crucial to students' mental health.
Restaurants can only offer delivery and take-out services in the shutdown zone and no indoor dining will be allowed.
Essential retail stores are capped at 50 per cent of their maximum capacity and non-essential stores are limited to 25 per cent.
The "emergency brake" shutdown begins this weekend and will last for at least a month.
Personal care services must also close.
On Saturday, the province reported 3,009 new cases and 16 additional COVID-19 related deaths.
Six public health regions, including Toronto's and Hamilton's, are now in the grey "lockdown" zone of the province's colour-coded system.
And despite being under shutdown since last November, Toronto, Canada's biggest city and the country's financial hub, remains Canada's biggest COVID hot spot.
The order imposes restrictions on businesses and residents across the Canadian province of 14.7 million people, beginning April 3.
On Thursday, officials reported more than 2,500 new cases of COVID-19, a almost 14% increase in hospitalizations and ICU admissions increasing dramatically.