The bomb went off at about 12:30 p.m. local time in a popular pedestrian zone in downtown Prague where there are many restaurants and cafes. The injured included tourists from Britain, the United States, Germany and Slovakia, Czech officials said.
"We can rule out it was a terrorist attack," police spokeswoman Iva Knolova told The Associated Press. "It was a criminal act," she said. She refused to elaborate.
The bomb attack likely targeted the owner of the Casino Royal, a police officer at the scene said on condition of anonymity. Earlier, police at the scene had said a hand grenade caused the blast.
The area was cordoned off and police were investigating the incident.
All of the injuries appeared to be slight, said Marek Uhlir, spokesman for Prague's emergency service. He said a child was among them.
The injured were taken to four hospitals in Prague for treatment, he said. He had no other details.
The Foreign Office in London said two British nationals were hurt in the explosion.
"British Embassy officials immediately went to the scene of the explosion and were heading to the local hospital to provide consular assistance," the Foreign Office said. "We are aware of two British nationals being injured."
The Foreign Office said relatives were being informed.
The U.S. Embassy was not immediately aware of any U.S. citizen injured in the explosion, spokeswoman Lisa Helling said.