Initial reports suggested that some people may be trapped in the rubble. The area around the explosion in Divadelni St has been sealed off by police.
Spokespeople for the fire brigade said that there had been at least one injury from the blast.
Police spokesman Tomas Hulan says it is not immediately clear what caused the blast in Divadelni Street on Monday morning.
A photo, tweeted by RT.com and credited to Digiolli on Twitter, shows the aftermath of the blast.
The street is covered with rubble and has been sealed off by police.
Jirina Ernestova, spokeswoman for the rescue service says rescuers expect some people are buried under the rubble. Firefighters spokeswoman Pavlina Adamcova says at least 13 people have been injured.
Windows in buildings located hundreds of meters from the blast were shattered, including some in the nearby National Theater.
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