OTTAWA -- A 24-hour strike at two of Canada's busiest postal sorting plants Tuesday forced delays in shipments of tens of thousands of letters and parcels across the country as Canada Post employees stepped up pressure to back their contract demands.

The one-day job action virtually shut down the Crown corporation's sorting hubs in the Greater Toronto Area -- the giant Gateway parcel facility in Mississauga, which processes roughly two-thirds of all parcels mailed in Canada, and the South Central mail plant in the city's east end.

The walkout came on day two of rotating work stoppages by members of the Canadian Union of Postal Workers that the union has threatened to continue until Canada Post sweetens its contract proposals for rural and urban carriers.

The union and postal service have been unable to reach new collective agreements for the two bargaining units in 10 months of negotiations.

On Monday, walkouts shut down postal operations in Victoria, Edmonton, Windsor, Ont., and Halifax, causing few delivery disruptions outside of those cities.

But Tuesday's job action in Toronto, where nearly 9,000 CUPW members walked off the job a minute after midnight, was expected to have a "significant impact" on Canada Post operations across the country, said corporation spokesman Jon Hamilton.

The agency said a "fair" estimate of delivery delays resulting from the walkout would range in the tens of thousands.

CUPW, which represents 50,000 postal employees, has called on Canada Post to address issues that have stemmed from the explosive growth of parcel deliveries, including health and safety concerns and precarious work.

The agency said Tuesday it has made overtures to the union with the aim of mitigating some of those concerns.

"We value the relationship with the union and have been able to find common ground on some issues and have also committed to work together constructively on several important files," Hamilton said in a statement.

"Those include working together to address employees workload concerns caused by parcel growth, additional financial services and going beyond pay equity for Rural and Suburban employees by extending job security and moving to one uniform for all delivery employees."

But a big issue for CUPW is the perceived overuse of temporary workers. The union wants Canada Post to provide greater job security through the creation of more full-time positions, arguing that temporary workers are consistently paid less, are not covered by health, dental and sick or disability insurance plans, have no guaranteed hours and cannot plan their futures.

Both sides have so far vowed to continue negotiating new agreements.

CUPW said its key demands also include an end to forced overtime, service expansion and equality between urban and rural and suburban carriers, known as RSMCs.

Toronto-area Canada Post employees were to return to work at midnight with rotating walkouts expected to continue Wednesday in other locations.