"This was not a terrorist attack, it was not a bomb," he said on Sunday at the scene of the explosion that injured at least 16 people.
Blazek said information so far showed a person threw a grenade at the pedestrian mall Na Prikope, just off Wenceslas Square in the tourist district of Prague at about 12.30pm (1030GMT).
The grenade rolled under a car which was parked near a restaurant and a casino before exploding, helping to limit the number of casualties, he added.
A police officer on the scene said it appeared the blast was related to criminal activity.
According to Czech news wire CTK, witnesses reported seeing a hand grenade being thrown by a young man from a white vehicle.
Most of those injured were guests sitting close by in the garden terrace of the Casino Royal restaurant outside the casino.
"This was not a terrorist attack, it was not a bomb"
Deputy Prague Mayor
Some of the injured were taken to four hospitals in the city.
More than 12 ambulances, together with police, firefighters and pyrotechnic experts were on the scene and police cordoned off the area.
Na Prikope is home to many shops, boutiques, casinos and restaurants and is usually packed with tourists.