Canada Post has locked out its 48,000 unionized workers across the country, saying that rotating 24-hour strikes have cost the company almost $100 million.

The announcement came just before midnight on Tuesday.

"We believe that a lockout is the best way to bring a timely resolution to this impasse and force the union to seriously consider proposals that address the declining mail volumes and the $3.2-billion pension deficit," Canada Post said in a statement.

The Canadian Union of Postal Workers had planned to continue its local strikes Tuesday night, targeting three smaller communities: Salmon Arm, B.C., Sioux Lookout, Ont., and Carbonear, N.L.

Some 15,000 postal workers in Toronto area and Montreal were picketing outside sorting stations earlier on Tuesday as the rotating strikes reached Canada's two largest urban centres, a move expected to have serious repercussions across the rest of the country.

By Tuesday afternoon, thousands of postal workers were walking picket lines across the Greater Toronto Area, waving signs and chanting their demands for better work benefits.

Picketers turned away trucks from competitors like UPS and FedEx, which were trying to deliver mail to Toronto's South Central sorting station.

Similar picket lines appeared overnight outside Montreal's Saint-Laurent sorting station, where striking workers blew whistles as they marched along a sidewalk outside the building.

Workers in Toronto and Montreal walked off the job shortly before midnight Monday. They were the most recent cities affected by a series of spot-strikes that have hit towns and cities across the country.

"The purpose of this action is to highlight the impact of service reductions in major urban centres," the union said in a statement.

Canada Post says it has lost nearly $100 million since the strikes began, much of it to competitors and to customers turning to online alternatives. The corporation says it has reduced delivery to three days a week because of the loss of business.

But striking workers say they are not worried about losing customers during the strike, claiming that is something management should consider as they stonewall negotiations and limit delivery to three days a week.

"If they don't want the losses, maybe they should tell the public that they are going to deliver their mail five days a week and should negotiate instead of waiting for the Harper government to legislate," said striking worker Irwin Nanda while picketing in Toronto.

When postal workers last went on general strike in 1997, they were legislated back to work after two weeks off the job.

This time round, strike action has so far been limited to localized 24-hour strikes in a variety of smaller communities and major centres including Edmonton, Hamilton, Regina, Winnipeg and Victoria.

Tuesday's walkout was the second to affect the 6,200 postal workers in the Montreal-area after they staged another 24-hour strike on June 6.

With files from CTV Toronto's Naomi Parness and The Canadian Press