TORONTO, ON — More than 620 flights, many of them international trips, have been canceled as the clock ticked closer Friday to , leaving travelers stranded around the world and scrambling during the peak summer travel season.

The bitter contract fight between Canada's largest airline and the union representing 10,000 of its flight attendants escalated Friday as the union turned down the airline's request to enter into government-directed arbitration, which would eliminate its right to strike and allow a third-party mediator to decide the terms of a new contract.

Flight attendants were poised to walk off the job around 1 a.m. EDT on Saturday. Around the same time, Air Canada said it would begin locking out the flight attendants from airports. The actions threaten to .

Air Canada said

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