TORONTO -- Mail service across Canada, which in normal times can be slow and erratic, is in disarray now as letter carriers hit one city after another without warning in a series of brief but sometimes bloody rotating strikes.

Police and strikers have been injured in scuffles in Calgary, Toronto and St. John's, Newfoundland, as the strikers attempted, sometimes successfully, to block mail trucks and the vanloads of replacement workers hired since the strikes started last Tuesday.

Striking letter carriers broke into a Montreal postal station last week, overturned mail racks, smashed chairs, trampled on letters and packages and beat up temporary employes brought in to replace them.

Sympathetic sorters and other unionized postal workers still on the job have told reporters that they are deliberately misdirecting some of the mail that is still going through.

Leaders of Canada's major unions have given strong backing to the Letter Carriers Union of Canada, which is resisting the government-owned Canada Post corporation's demands to scale back the work force and alter overtime and work rules. Faced with a $100-million-a-year operating deficit by Canada Post, the government of Prime Minister Brian Mulroney has taken a tough line in negotiations.

"Ronald Reagan broke the air traffic controllers because the labor movement in the United States stood by and let it happen, but it's not going to happen here," Bob White, president of the powerful Canadian Auto Workers, said as he walked the picket line with letter carriers in Toronto on Saturday.

The letter carriers, who have adopted the tactic of rotating strikes partly because they do not have a strike fund, appeared to win one round over the weekend when postal management officials, clearly concerned about the fisticuffs and sabotage, called for a federal mediator and agreed to end the experiment of hiring replacement workers to help deliver the 25 million pieces of mail handled each day nationally.

Harold Dunstan, Canada Post's chief negotiator, said Friday that management employes would attempt to fill in if strikes continue.

The unreliable service and years of labor turmoil have long made the government corporation an object of national derision. "It's a prehistoric monster," Elizabeth Kriegler, a vice president, said last week.

Many businesses, and even federal government agencies, have long since given up on Canada Post for their most important communications. The private courier business flourishes in Canadian cities. Last month, postmasters in border U.S. prairie towns said they were getting more and more business from Canadians who were renting post office boxes.

"People can shave five or six days off their deliveries," said Dale Anderson, postmaster of Walhalla, N.D., which borders Manitoba.

A federal committee that reviewed Canada Post reported in 1985 that the U.S. and Canadian systems were roughly equal in productivity 15 years ago but by 1984, the U.S. output outstripped Canada's by more than 50 percent.

In 1984, letter carriers were absent an average of 17 days yearly because of illness or injury, compared with fewer than six days for employes in commercial mail businesses. Letter carriers are paid to service a route but if they finish it before eight hours are up, they may then fill in for absent workers, at overtime rates of time-and-a-half.

Since 1981, Canada Post has decreased its annual operating deficit, which had been as high as $440 million, mostly by raising postal rates. The cost of a first class stamp, now about 29

U.S., more than doubled between 1981 and 1985.

Mulroney's government has been trying to balance the books by closing hundreds of rural post offices and eliminating home delivery in some new suburban areas.

For months, controversy has raged as the government has attempted to sell a skeptical public on the idea of one big common "super mailbox" for rural areas and some subdivisions. Canada Post has also begun to "franchise" out post offices to drugstore chains and others who use their own employes, at wages much lower than those of unionized postal workers.

The federal government has declared it will go to the mat with the letter carriers to achieve further economies. Harvie Andre, minister in charge of postal services, told the House of Commons last week that the goal of financial self-sufficiency has been postponed too many times for the government to give in now.