Denver Police diverted traffic from Colfax Avenue at Broadway as they investigated an accident involving a pedestrian. Colfax reopened about 5:15 p.m.
Raquel Lopez, a spokeswoman for the department, said the accident happened just before 4 p.m.
The victim was transported to a hospital with serious injuries.
The driver of the vehicle remained on scene.
According to Ruth Kanatser, an employee at the Harm Reduction Action Center who winessed the incident, a brown Chevrolet Impala heading south on Broadway struck the pedestrian as it turned west onto Colfax Avenue.
"A bunch of us ran up, lots of people were screaming 'don't move him, don't move him,' and I started trying to get traffic out of the way," said Ruth Kanatser, 33.
The pedestrian, described as an elderly man with a slight build, was bleeding from his head and semi-conscious when transported by ambulance, according to witnesses.
Police are investigating how the accident occured, but no information is available yet about who was at fault.