All five lanes of the highway in each direction were blocked and will be closed for the foreseeable future, officials said.
That prospect sent shudders through metro Atlanta commuters, who already contend with heavy traffic. An estimated 250,000 vehicles drive daily through that stretch of I-85, according to the Georgia Department of Transportation.
Cortez Stafford, a spokesman with the Atlanta Fire Department, said a unit was returning from another fire when it saw heavy smoke coming from under I-85.
"There was a 40-feet or higher wall of fire. Power lines were falling and arching heavily and falling in the streets," Stafford told CNN.
The elevated span of highway -- a few miles north of downtown -- collapsed about 7 p.m. as crews battling the fire got out of danger. The cause of the fire was not immediately known.
As concrete began falling from under the bridge, firefighters were asked to step back, Stafford said. "Not even two minutes later, the highway fell with a big 'kaboom.' (It) knocked our guys back," he said.
There were no injuries.
In an unusual move, two fire trucks from Hartsfield-Jackson Atlanta International Airport in south Atlanta came to the scene and sprayed foam on the fallen section and flames.
Interstate traffic was stopped and turned around, creating jams that extended 5 miles or more, not long by Atlanta standards, but many drivers were stuck in the area for hours.
Georgia Department of Transportation spokewoman Natalie Dale told CNN, "Given the collapse on the northbound side, I cannot fathom the southbound will be open anytime soon."
Dale said inspectors were waiting for chance to inspect the freeway. Transportation authorities are trying to put together a traffic plan, a detour plan for commuters.
"We will be up all night and into the morning," says Dale. A state highway runs next to the section of the interstate, but it is just two lanes.
Gov. Nathan Deal told reporters the company that originally built the elevated highway would assess the damage.
"It's going to take some time to get it repaired and to get it back in service," he said, without offering a time frame for reopening.
The fire occurred near the city's busy Piedmont Road and just south of Georgia 400, a major route to and from the northern Atlanta suburbs.
CNN's Eliott C. McLaughlin was driving north on I-85 during evening rush hour when he saw black smoke rising from underneath the elevated highway.
Cars getting off at an exit for 400 started slowing more quickly than a normal rate and some cars pulled off onto the shoulder.
But many cars on the left side of the five-lane section barreled through the thick black smoke. They disappeared into the darkness as they drove on, he said.