The incident happened just before 6:30 a.m. local time in Tempe, located east of Phoenix. The Tempe Police Department said on Twitter that officers and the Tempe Fire Department were on the scene of a train derailment and "large fire" over Tempe Town Lake.
"Several roads are restricted near Tempe Town Lake and west of downtown Tempe," the department said. "Please avoid the area."
Tempe Fire Medical Rescue told Fox News that multiple train cars were on fire, some carrying lumber. Several agencies are assisting what was described as a second-alarm fire.
Fire and smoke could be seen along the length of the train bridge, including a portion that collapsed in a park area.
Aerial images from FOX10 showed the fire continuing to burn as first responders arrived at the scene.
None of the train's crew members were hurt but there was a report of someone suffering from smoke inhalation, Tim McMahan, a spokesman for Union Pacific Railroad, told the Associated Press.
Some of the train's cars carried lumber and others were tanker cars.
McMahan said he did not know what was inside the tanker cars but that there were no reports of any leaks.
Camille Kimball, who was riding her bike under the bridge before it collapsed, told CBS 5 the noise of the train got extremely loud and as she came off the bridge.
"I turned around to look and got the fright of my life," she told the television station. "Now there's fire pouring into the lake from the middle of the bridge. ... It' looks like a scene from hell, truly. A scene from hell. ... The flames are intense and the sky is filled with black smoke."
Multiple videos from the area showed the bridge partially collapsed.
Tempe Fire Medical Rescue told Fox News that no injures were reported due to the derailment and subsequent fire.
Tempe Police Chief Sylvia Moir urged people to avoid the area, tweeting that the scene was "very dangerous."
"Stay out of Tempe Town Lake and away from the area," she said.
Valley Metro, who operates the light rail system in the Phoenix area, said due to the incident involving the freight train, Valley Metro trains were prevented from crossing the Tempe Town Lake Bridge.
Buses were being arranged to transport passengers through the area.
Tempe Town Lake was closed until further notice due to the derailment, according to police.
The 2.5-mile Tempe Town Lake is is a popular recreation spot for jogging, cycling and boating and close to Arizona State University.
Fox News' Stephanie Bennett in Phoenix and the Associated Press contributed to this report.