The force of the explosion pushed up four manhole covers. Witnesses reported seeing heavy smoke coming out of the street in the 200 block of South Hill Street and several other nearby locations when the incident occurred at 12:30 p.m.
Firefighters responded and said there were no reports of injuries.
Elaine Balden of Pasadena was stopped at a red light downtown when she saw the explosion nearby.
"All of a sudden, the street explodes," she said. "Literally the manhole cover went sky high, flames coming out of the street. Very scary. I turned around."
About 50 residents were without power for a short time.
The cause of the explosion is under investigation.