The blockade of GM's Canadian headquarters in Oshawa, Ont., continued Saturday. (Aaron Harris/Canadian Press)

General Motors workers shut down the company's Oshawa, Ont., car and truck plants for several hours Saturday during a protest against plans to close a truck assembly line.

About 100 people took to their cars and drove in a slow convoy around the plants, blocking the delivery of parts, union spokesman Keith Osborne said. Federal politicians, including Liberal Leader Stéphane Dion and NDP Leader Jack Layton, were present to show their support.

Transport workers could be heard honking their horns in support of the workers.

Osborne, the Canadian Auto Workers' plant chair for GM's Oshawa complex, said the vehicles travelled at about 10 km/hr in a protest aimed at pressuring the company to reverse its decision.

The workers' blockade of the automaker's Canadian corporate headquarters continued for a fourth straight day. It began Wednesday, a day after GM announced plans to close four plants in North America. The Oshawa truck-assembly line is slated to shut down next year, putting 2,600 people out of work.

The announcement came just two weeks after members of the Canadian Auto Workers union ratified a three-year contract with General Motors, which they say contained a promise the plant would build a new hybrid truck that would give them work until at least 2011.

The union, which said it'll continue the blockade, contends the Oshawa plant shutdown violates that contract, in which members agreed to a wage freeze and several concessions to maintain the jobs.

GM will also close its Windsor transmission plant in the third quarter of 2010, ending another 1,400 jobs.

Union president Buzz Hargove met Friday with GM brass at the company's headquarters in Detroit, but GM flatly refused a union demand to keep the truck plant open past 2009.