Greek police used fire extinguishers and batons against migrants on the island of Kos, as tensions boiled over in a sports stadium where hundreds, including young children, had gathered to wait for immigration papers.
The scuffles took place a day after a police officer on the island was suspended for brandishing a knife and slapping a man identified by Greek media as a Pakistani migrant.
Hundreds of migrants ran from clouds of thick white powder set off from fire extinguishers wielded by police officers who were apparently trying to disperse the crowd.
It was not immediately clear what had sparked the violence.
Greece has seen a surge in the number of migrants and refugees, fleeing war zones like Syria, trying to enter the European Union by crossing in rubber dinghies from Turkey.
The United Nations refugee agency UNHCR said 124,000 had landed on Greek islands this year.
The UNHCR called conditions for migrants on Kos and other islands "shameful".
Kos Mayor Yorgos Kyritsis said strained local services, including the police and coast guard were unable to cope with the influx.
"This situation on the island is out of control," he told Greek TV. "There is a real danger of uncontrollable situations. Blood will be shed."
A Greek police official said riot police squads would be deployed to the island and police officers from the neighbouring islands of Rhodes and Syros would be sent in as reinforcements.
