PRAGUE (AP) — A powerful explosion badly damaged an office building in the center of the Czech capital Monday, injuring up to 40 people. Authorities believe people may still be buried in the rubble.

It was not certain what caused the blast in Divadelni Street, in Prague's Old Town, at about 10 a.m., but it was likely a natural gas explosion, police spokesman Tomas Hulan said. Tourists at the famed Charles Bridge also felt the blast.

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

Zdenek Schwarz, head of the rescue service in Prague, said up to 40 people were injured, at least four of them seriously.

Rescue service spokeswoman Jirina Ernestova said there were foreigners among the injured but had no further details immediately.

Some of the injured were taken to Prague's hospitals for treatment while others, many of whom were hit by flying glass, were treated by rescuers at the scene.

Firefighters spokeswoman Pavlina Adamcova said rescuers were still searching the rubble, using sniffer dogs.

Adamcova said two or three people were still missing.

Windows in buildings located hundreds of meters from the blast were shattered, including some in the nearby National Theater.

"There was glass everywhere and people shouting and crying," Vaclav Rokyta, a Czech student, told the AP near the scene.

"I was in the bathroom, no windows, the door was closed. Honestly, if I had been in my bed I would have been covered in glass," said Z.B. Haislip, a student from Raleigh, North Carolina, who was in a nearby building.

The Faculty of Social Sciences of Prague's Charles University and the Film and TV School of the Academy of Sciences of Performing Arts are located next to the damaged building.

The road closures caused major traffic disruption and confused thousands of tourists. Some new arrivals to the city had to stand on street corners, unable to reach their hotels, their baggage loaded onto trolleys. Hotel staff urged them to be patient.

Prime Minister Petr Necas said in a statement he was "deeply hit by the tragedy of the gas explosion."