She said there were fears three people may be buried in the rubble of the damaged building which police said was a former block of flats now used as office space.
Ms Ernestova said ambulances were staying at the scene "in case someone was pulled out of the rubble".
Emergency services chief Zdenek Schwarz told Czech TV four people had suffered serious injuries, while other Czech media said up to 55 may have been hurt.
An AFP photographer at the scene saw dozens of people with cuts, likely sustained by shattered glass from windows along the debris-covered street.
"A gas blast seems to be the most likely cause. The explosion was rather massive and damaged windows in several streets," Mr Hulan said.
Czech media quoted witnesses as saying they could smell gas in the street.
"Blast in Divadelni (street), many injured. Windows broken at journalism faculty," Milos Cermak, a Czech journalist who was lecturing in an adjacent university building at the time of the blast, wrote on Twitter.
"Police say gas still leaking. Helicopter above us. Classes cancelled for rest of week," he added.
Ms Ernestova said ambulances were staying at the scene "in case someone was pulled out of the rubble".
Emergency services chief Zdenek Schwarz told Czech TV four people had suffered serious injuries, while other Czech media said up to 55 may have been hurt.
An AFP photographer at the scene saw dozens of people with cuts, likely sustained by shattered glass from windows along the debris-covered street.
"A gas blast seems to be the most likely cause. The explosion was rather massive and damaged windows in several streets," Mr Hulan said.
Czech media quoted witnesses as saying they could smell gas in the street.
"Blast in Divadelni (street), many injured. Windows broken at journalism faculty," Milos Cermak, a Czech journalist who was lecturing in an adjacent university building at the time of the blast, wrote on Twitter.
"Police say gas still leaking. Helicopter above us. Classes cancelled for rest of week," he added.