The blast, the cause of which was not immediately known, ocurred opposite a a square in Kocaeli where the demonstration was taking place, the city's police chief Huseyin Namal told the Anatolia news agency.

"We do not know yet what caused the explosion," he said. "Three people suffered minor cuts on their bodies."

Bomb experts are investigating the blast, he added.

A young girl was among the injured, Anatolia reported.

The demonstration, organised to condemn mounting violence by the separatist Kurdistan Workers' Party (PKK), continued after the incident, the agency said.

Turks have taken to the streets since last week when a PKK ambush near the Iraqi border left 12 soldiers dead.