Updated at 5:15 p.m. ET: A big piece of a bridge being demolished fell unexpectedly Saturday during the construction project known as "Carmageddon II," but work was only temporarily halted.
Officials on scene said a portion of the Mulholland Bridge overpass fell about 4 p.m. local time, but no one was injured.
Up to that point, other than having to chase some motorists, skateboarders, and walkers off a 10-mile stretch of Interstate 405, few problems were reported in the early hours of "Carmageddon II," officials said.
The stretch of one of the world's busiest freeways was shut down early Saturday, and if all goes according to plan it'll stay that way until the bridge is rebuilt before the Monday morning traffic crunch.
Dave Sotero, a spokesman for Metro, the agency overseeing the $1 billion widening of the San Diego Freeway, said that it's not clear what caused the large chunk of the bridge to fall.
The chunk fell from the eastern span of the bridge onto the slope leading down to the edge of the freeway.
"During the demolition of a huge bridge like this, it's not unusual for pieces of all sizes to come down," Dan Kulka of Kiewit Infrastructure West Co., told NBCLosAngeles.com. "Although we didn't anticipate this large of a piece to come down, this is certainly not unusual.
"As soon as it happened we stopped and … had our structural engineers analyze it and redevelop the plan. And now we will continue to demolish it," he added.
The work is to widen the bridge itself to expand the freeway below.
Los Angeles Mayor Antonio Villaraigosa had said Saturday afternoon that work was one hour ahead of schedule.
For weeks drivers have been warned to stay away from the segment of Interstate 405 that is now shuttered through the Sepulveda Pass on Los Angeles' west side for the entire weekend.
If drivers don't avoid the area, officials warn, a city-wide traffic jam could result. But beyond just scare tactics, city officials have been encouraging Southern Californians to get out and enjoy their own neighborhoods on foot, on bikes or via short drives on surface streets.
During a similar closure last year commuters stayed away from the freeway in droves, the shutdown was considered a success, and crews finished the first phase of the work early.
This time, the contractor faces a penalty if the work isn't done in 53 hours: $6,000 per lane of freeway per 10 minutes.
Watch live video from NBCLosAngeles.com of bridge demolition:
Meanwhile, TV news crews have a plan to avoid a traffic jam in the sky as they cover the shutdown.
Residents complained of low-flying, noisy helicopters hovering nonstop over the region last year.
Watch video report from NBCLosAngeles.com as Carmageddon 2 officially begins:
"It was constant," Richard Close, president of the Sherman Oaks Homeowners Association, whose members live in many of the homes closest to the freeway, recently told The Associated Press. "It was a combination of the news media paparazzi and tour operators taking people who wanted to get a picture of the 405."
Although the area gets its share of paparazzi helicopters because of Charlie Sheen and other celebrities who live in the area, Close said they usually go away when the sun sets. During Carmageddon, however, the area is brightly illuminated overnight so construction workers can safely do their jobs.
This time, local television news directors have plans to pool coverage by using video from a single helicopter making limited flights over the freeway, according to Rick Terrell, executive director of the Radio & Television News Association of Southern California.
NBCLosAngeles.com as well as Reuters contributed to this report.
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