Five people were treated for minor injuries and rescuers were trying to reach one injured person inside the residential building in Vienna's 15th district, said police spokeswoman Adina Mircioane.
"We are also bringing in sniffer dogs to search for other people who might be trapped inside," Mircioane said, adding that the cause of the explosion wasn't yet known.
Witnesses said they heard a dull thud before the top two floors of the building collapsed, raining debris onto the street shortly after 10 am local time today.
A video taken by Knops showed a vast plume of dust enveloping the street moments after the incident. The video was reviewed by The Associated Press and along with Knops' description appeared consistent with the police account.