Traffic was heavily impacted for hours, with cars sitting bumper-to-bumper. 4th Street was initially closed from Mission to Howard, but has since reopened. Folsom, however, is still closed from 3rd to 5th streets.
The gas leak emptied out the Moscone Center and buildings surrounding a 76 Gas Station on 4th and Folsom streets. Police cordoned off a one block area.
The Moscone Center was hosting a medical convention when alarms sounded throughout the building. Barney Kijek immediately thought of an attack.
"Just below the surface everybody is a little edgy because of stuff like Boston," Kijek said. "Particularly when you're in a group, I mean, there's ten thousand people at this convention."
Todd Noe, who was also at the convention, says that there wasn't a panicked exit from the auditorium.
"Everybody was pretty calm when the alarm went off, we all just slowly walked to the escalators and got outta there, pretty simple," he said.
The leak happened at about 12:30 p.m. Tony Villarreal was just leaving his job at the Moscone Center when he smelled gas.
"I was like, dude, you guys smell that?" Villareal said. "We should be gone, we should be off this block as far as I'm concerned. I mean, a gas leak if it blew, who knows how many blocks it can take out."
Police had the same concern. An evacuation was ordered for buildings in the area.
"We did evacuate several hundred people at the Yerba Buena Gardens, at the Moscone Center, Convention Center, Museum of Modern Art, apartment buildings surrounding the area," SFPD Officer Albie Esparza said. "Basically all the affected people were asked to go to St. Patrick's Church where the Red Cross is there assisting them.
Other people were told to move to 4th and Mission streets until the danger had passed.
Police say a private construction crew hit the gas line. PG&E crews were brought into the area at about 2:30 p.m. with a police escort. It was the only way to get them through the traffic that had been snarled there for hours.
Those PG&E crews have capped that line. They plan on going through buildings in that are to make sure there hasn't been gas collecting inside, posing any kind of danger.
The police are asking all drivers to stay away from the area. They're unsure when all streets will be open and traffic is still heavily impacted.