A police spokesman said a blast in the Czech capital Prague was probably caused by gas and that there had been about 15 people in the building.
There have not been reports of any deaths, a fire department spokeswoman said.
The explosion, close to Prague's National Theatre, was heard as far as Prague Castle about a mile away.
Windows in neighbouring buildings were blown out, including Prague's landmark Cafe Slavia.
More top news
Cricket stars, celebrities and fans have joined a social media campaign paying tribute to Phil Hughes by leaving their cricket bats out.
Eight out of 10 asthma sufferers are not getting enough clinical care, a leading health charity has found.
Phillip Schofield has revealed that an eccentric fan who wanted to marry him arranged a wedding - leaving him with the bill.