The leak was capped by 7:15 a.m., and the 1st Avenue South exit from the West Seattle Freeway has been reopened. South Spokane Street was later reopened between 1st Avenue and East Marginal Way.
Seattle Fire Department spokesman Kyle Moore says the leak was in a two-inch gas line and it pumped a huge volume of natural gas into the area for about an hour and a half.
Train traffic also was temporarily diverted from the area, and a warehouse building at 55 S. Spokane St. was evacuated for a time.
Moore said the break was in a major line serving an industrial compex. Witnesses earlier reported a very strong odor of natural gas in the area.
Moore says the street closures were necessary to prevent sparks from vehicle traffic from igniting the combustible gas.
Puget Sound Energy crews later determined that the leak was caused by something falling on the gas meter.
"It looks like something fell on the meter and broke it and detached it where the pipe goes into the ground," said PSE spokesperson David Namura.