OTTAWA—Canada’s unionized postal workers walked off the job, halting handling of almost all mail and parcels and threatening lengthy delivery delays heading into the busy holiday season. The strike by tens of thousands of Canadian Union of Postal Workers members began early Friday after a year of contentious bargaining failed to yield a collective contract agreement for mail carriers.

The union walked out after talks with the country’s main mail operator failed. The stoppage could disrupt deliveries before the holidays.

Canadians may face delays in getting their holiday gifts and purchases this year after workers at the national postal service went on strike.