Thousands of frustrated Canada Post employees went back to work Monday, but not before staging protests across the country to vent their frustration at Ottawa's handling of labour unrest at the Crown corporation.

Unionized Canada Post employees and other labour groups held a noisy rally in downtown Vancouver Monday, with many carrying placards emblazoned with messages criticizing the Conservative government.

"They made it clear that they don't really care about our wellbeing," said CUPW member Mike Palesek. "And we'll return the favour."

Under the terms of federal legislation passed on Parliament Hill over the weekend, Canada Post had to end its lockout and 48,000 urban postal workers had to return to work by 8:45 p.m. ET Monday.

Officials with the Canadian Union of Postal Workers said their members would abide by the legislation. But they did not go cheerfully.

There was a brief confrontation at a sorting plant in Halifax Monday morning when about 100 unionized workers, upset at the federal legislation, turned away managers as they tried to enter the building. Police were called in and the managers were eventually allowed in at 4 p.m. local time.

In St. John's, Newfoundland and Labrador, workers held a rally outside a Canada Post building before heading back to their jobs Monday afternoon.

In his address to the crowd, the NDP MP for St. John's South-Mount Pearl, criticized Ottawa's handling of the labour dispute.

"You cannot go for billions of dollars of cuts and savings from the workers of this country," Ryan Cleary told the crowd. "The next Conservative action plan cannot be your pension."

Canada Post employees in Edmonton also held a protest before resuming their duties, while their counterparts in Calgary continued to march on the picket lines until late Monday afternoon.

The back-to-work legislation forces postal workers to accept wages that are less than what Canada Post had last offered. It will dictate non-wage issues by choosing between a final offer from both Canada Post and the union.

"It's all favoured towards Canada Post management and it's not going to be good for us," said George Floresco, third national vice-president of the Canadian Union of Postal Workers.

Mail delivery could begin to resume as early as Tuesday, although Canada Post officials said it may take quite a while for service to get back to normal.

The Crown corporation began unsealing mailboxes Monday in preparation for resuming service. Post offices are also set to reopen Tuesday, with mail already in the system taking priority over newly mailed items.

Union vows not to forget

Meanwhile CUPW says it won't soon forget what it calls the Conservative government's "anti-labour" actions.

Jeff Callaghan, national director for the union's Atlantic region for the union, said his members are keen to return to their routes but plan to protest Conservative MPs who voted to force them back to work.

"What we're going to be doing over the summer months is visiting the Tory MPs who were quite quick to take away our rights," Callaghan told CTV News Channel on Monday afternoon. "Some of the discussions in the (House of) Commons referred to (postal workers) as less than Canadian."

Urban postal workers began a series of rotating strikes on June 3, after failing to come to a contract agreement with Canada Post. Wages for new employees, sick days and the company's plan for the future were among the sticking points between the Crown corporation and its employees.

In the early days of the rotating strikes, the post office and the union remained engaged in negotiations. But the two sides didn't get very far and Canada Post announced June 14 that it was locking out its striking workers.

A day later, Labour Minister Lisa Raitt signalled that the government would table back-to-work legislation if the two sides couldn't reach a deal. The bill was tabled on June 20 and led the opposition New Democrats to hold a days-long filibuster -- a non-stop debate in the House of Commons -- to hold up passage of the back-to-work bill for 58 hours.

But they could only stall the bill for so long and it passed, was approved by the Senate and received royal assent during the same weekend.

Canada Post spokesperson Jon Hamilton said postal workers will begin sorting through the mail that has accumulated during the strike and accompanying lockout when they return to work on Monday.

"What we're going to do is move as quickly as possible to get the mail restored across Canada," Hamilton said Monday morning.

Once sorting begins, he said postal workers will be able to begin distributing the mail as soon as Tuesday afternoon.

Denis Lemelin, the president of the Canadian Union of Postal Workers, has said union members will obey the back-to-work order from the government.

But he said CUPW will examine the back-to-work legislation that passed Sunday to see if it has any holes to exploit -- and the union will also take an aggressive stance during the upcoming arbitration process.

"The message of the government is the fact that if people want to stand up for their rights, they will legislate you back, and they will impose the wages," he told CTV News Channel on Monday. "It's a signal to all the people in the federal public sector (that) if they don't like the wages and they go on strike, they will be legislated back."

Callaghan says he knows some Canadians might see his union's demands as somewhat lofty in an era of cutbacks and economic uncertainty. But he says they didn't come out of thin air.

"What Canadians should remember is that what we have in our collective agreements has been fought for… since the 1960s," he said. "This wasn't all given to us."

With reports from CTV Vancouver's Bhinder Sajan, NTV's Pam Parsons and files from The Canadian Press