A massive fire has caused a bridge on the Interstate 85 road to collapse in the US city of Atlanta, completely shutting down the heavily-travelled highway.
Georgia governor Nathan Deal has declared a state of emergency, saying the state is mobilising resource to try to keep traffic disruption to a minimum while emergency work continues.
Georgia's top transportation official said there is no way to tell when the highway, which carries 250,000 cars a day, can be safely reopened to traffic in either direction following the collapse, which happened during afternoon rush hour.
"We will have to continue to evaluate the situation and adjust as we do," transportation commissioner Russell McMurry said.
"This incident - make no bones about it - will have a tremendous impact on travel."
The interstate is a major artery for the US South and a thoroughfare for traffic heading north and south through Atlanta.
The bridge collapse effectively "puts a cork in the bottle," Georgia state patrol commissioner Mark McDonough said.
The Metropolitan Atlanta Rapid Transit Authority plans to increase rail services and have additional staff on hand to help passengers reach their destination.
The incident began with a fire which The Atlanta Journal-Constitution said burned for more than an hour under I-85 northbound near Piedmont Road, spewing clouds of black smoke skyward.
The interstate has been closed indefinitely and the department of transportation warned all motorists to stay off I-85.
Traffic was bumper-to-bumper on nearby surface streets on Thursday night as people scrambled to find alternate routes.
However, officials said no-one was hurt, despite dramatic images of towering flames and plumes of smoke.
"This is about as serious a transportation crisis as we can imagine," Atlanta mayor Kasim Reed said.
All Rose Diggs told The Atlanta Journal-Constitution that she lives less than a mile from the fire site but could not get home because of blocked surface streets.
She said she was told to walk despite being disabled.
Capt Mark Perry of the Georgia State Patrol said the cause of the fire beneath the bridge is unknown, but terrorism is not suspected.
Mr Deal told reporters that some PVC plastic materials in a vehicle may have caught fire.
"I do not know why they did or what the source of their transport was," he said.
"But those are questions that will hopefully be answered at least by tomorrow morning."
Atlanta Fire Department spokesman Sgt Cortez Stafford said no cars were on the overpass when it fell.
"Our guys got here quickly and shut down the interstate and said: 'No-one else is driving over this bridge'."
Firefighters noticed chunks of concrete falling from the bridge and got out of the way just minutes before it collapsed, Sgt Stafford said.
Mr Deal said inspectors are at the scene and they have contacted the original company which built the bridge to come in and assess the extent of the damage.
"We're trying to determine everything we can about how quickly can we repair it and get it back in service," Mr Deal said.
"I can assure you we will do everything to expedite the repair and replacement of that section of the bridge."