Rescuers had feared there could be bodies trapped under the rubble of the heavily damaged office block, but the city's mayor said nobody was believed to have been killed.
The force of the explosion damaged parked cars and two adjacent university buildings, blew out windows in nearby streets, and shook apartment blocks across the Vltava river.
"It was a gas blast, not a terror attack," Prague mayor Bohuslav Svoboda told reporters.
He added that "they haven't found anyone (in the rubble)... And nobody is reported missing".
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"Everyone was running, the blast toppled the bookshelves in the library. In one classroom there were lots of injured students, the librarians had blood on their T-shirts," she added.
"At one department the blast wave broke a window on one side and warped the door on the other so people couldn't get out and had to kick the door open."
Forty-three people were injured, including two Kazakhs, two Portuguese, one German and one Slovak, said Zdenek Schwarz, head of Prague emergency services.
An AFP photographer at the scene saw dozens of people with cuts from shattered glass from windows along the debris-strewn Divadelni Street.
Injured victims were treated on the spot for cuts, some with blood streaming down their faces and bandages on their heads and many of them in shock.
Jan Hora, a structural engineer, told Czech TV the damaged building -- a former block of flats now used as office space -- had to be supported with poles but would probably not collapse.