A powerful blast in the Czech capital, Prague, has injured dozens of people and possibly trapped many others inside a building damaged by the blast, emergency officials said.

Monday's incident, which appeared to have been caused by a natural gas explosion, tore through a building near the National Theatre and blew out windows in neighbouring establishments, said Tomas Hulan, a police spokesman.

"A gas blast seems to be the most likely cause. The explosion was rather massive and damaged windows in several streets," Hulan said.

Up to 40 people were injured, according to Zdenek Schwarz, head of the rescue service in Prague.

A Reuters witness at the site said about a dozen people were being treated by emergency services.

Talking to Al Jazeera, Frantiske Bouc, business editor of the Lidove Noviny newspaper said people were "scared and shocked", adding that Prague "is not used to explosions".

It was not clear how many people were  trapped in the building.  

"There is an unknown number of injured and trapped people," said Jirina Fikejzova, Prague emergency services spokeswoman.

The street was covered with rubble and sealed off by police who also evacuated people from nearby buildings and closed a wide area around the explosion site.

Witnesses said the blast blew out windows in Prague's landmark Cafe Slavia, and was heard as far as Prague Castle about 1.6km away across the Vltava river.

Czech media quoted witnesses as saying they could smell gas in the street.

Source: Al Jazeera And Agencies