TORONTO (AP) _ Letter carriers stayed off the job today across the four Atlantic provinces, in parts of Quebec and in Vancouver as Canada's first major postal strike in six years entered its third day.

One policeman was injured in a clash between police and strikers in St. John's, Newfoundland.

Workers continued to strike in Edmonton and Victoria for the second day, but letter carriers in Montreal and Calgary, where the strike began Tuesday, returned to work.

Meanwhile, Canada Post planned to present a new contract proposal to the 20,000-member Letter Carriers' Union of Canada today in Ottawa. The main issue in Canada's first letter carriers' strike in 19 years is job security.

Neither side was optimistic the proposal would end the strike.

''The only thing different will probably be the paper it's printed on,'' union spokesman Mike Villemarie said. ''Maybe not even that. It could be a photo copy.''

Harold Dunstan, general manager of labor relations for Canada Post, said the agency could not change its bargaining position significantly and still meet the government's goals.

Canada Post, a crown corporation established in 1981, is under orders from Prime Minister Brian Mulroney's Conservative government to erase its operating deficit of $99 million by March. It wants to eliminate 8,700 union jobs and close hundreds of rural post offices.

The corporation has asked letter carriers to accept a pay freeze, longer delivery routes, a lower wage scale for new employees, and the elimination of several job protection measures contained in the previous contract, which expired Dec. 31.

The union wants to preserve the job protection measures and job guidelines that guarantee overtime pay for carriers who make extra deliveries after completing their normal rounds in fewer than eight hours.

The union also wants an unspecified wage increase to cover Canada's 4.5 percent inflation rate. Carriers earn an average of $10.07 an hour.

The postal agency is trying to maintain daily mail delivery to business customers and get mail out twice a week to residential customers by using strikebreakers. The union says it is falling far short of its goal.

The union was rotating the job action and said that could lead to a full walkout. The union chose sporadic strikes because it does not have a strike fund.

In Halifax, a standoff between striking letter carriers and police continued today outside a major postal sorting station.

More than 100 of the 250 postmen began blocking entrances to a sprawling plant Wednesday night and police threatened mass arrests at one point before backing off several hours later.

Picket lines also went up today at postal stations in Vancouver, where 1,400 letter carriers were on strike.

Hundreds of workers were reported off the job in New Brunswick, Nova Scotia and Newfoundland. In St. John's, Newfoundland, pickets broke the windshield of a truck attempting to enter a postal plant. No arrests were made, and one policeman was injured.

A union spokesman said about 890 workers remained off the job in Edmonton and 370 in Victoria. About 3,500 workers in Montreal and just over 900 in Calgary were returning to work today, but a dispute over lunches was blocking the return of 54 others in Cornwall, Ontario, the spokesman said.

In Vancouver, one truck driver brandished a baseball bat at taunting pickets as he drove into the main depot, but no violence was reported.

Canada's last postal strike involved sorters and clerks, who stayed off the job in 1981 for 42 days.