Prime Minister Justin Trudeau took a hard line against trucker protests that have paralyzed Canada’s capital city and have now halted commercial traffic to the U.S. at the busiest border crossing between the two countries.

The Ambassador Bridge that connects Windsor, Ontario, with Detroit was shut down in both directions late Monday, according to a Canadian government website and the Michigan transportation department. It’s the most important land crossing for goods between Canada and its largest trading partner and a crucial artery for auto parts suppliers and manufacturers.

About 1.4 million trucks entered the U.S. through Detroit last year, almost all of them via the bridge, according to data from the U.S. Department of Transportation and an association of bridge and tunnel operators. Protesters on tractors and trucks blocked access to another major U.S.-Canada bridge about 65 miles (105 km) northeast of Detroit on Sunday.