Chaos has broken out on the Greek island of Kos amid attempts to relocate hundreds of migrants to a football stadium for registration.
Police officers used batons and sprayed fire extinguishers as they tried to impose order on the crowds.
It comes after an officer on Kos was suspended for slapping one man while brandishing a knife.
Authorities are struggling with a rapidly growing number of migrants who have arrived hoping for a better life.
Many migrants and refugees have been camping alongside roads and beaches on the Aegean island, a popular tourist destination.
But an attempt relocate hundreds of people to a stadium for registration on Tuesday degenerated into chaos, with scuffles breaking out in the long queues.
“The situation on the island is out of control,” Kos mayor Giorgos Kiritsis told the Greek news agency ANA. “There is a real danger that blood will be shed.”
Protesting migrants demanding quick registration also began blocking the main coastal road in the island’s main town, according to the Associated Press.
They reportedly chanted: “We want papers, we want to eat!”
Greece has been overwhelmed by the number of migrants and refugees arriving, with at least 124,000 people reaching its shores by sea in the first seven months of this year alone, according to the the United Nations refugee agency.
The UNHCR says nearly all new arrivals in Greece are refugees from the wars in Syria, Iraq and Afghanistan.
Its European Director, Vincent Cochetel, has said facilities on the Greek islands are “totally inadequate”.
A policeman on Kos was suspended on Monday after he was caught on camera striking a migrant.
The footage shows him roughly pushing back migrants outside a local authority building in Kos, slapping one man across the face as he shoves others, telling them to get back behind a line he has drawn on the pavement with the knife.
Greek police say 156,726 migrants were arrested for entering or remaining in the country illegally from January to July 2015, compared with 32,070 for the same months in 2014.
Meanwhile the local coastguard said it had rescued 329 migrants in seven separate incidents off the coast of Lesbos and Kos in the 24 hours from Monday morning, according to AP.
The islands are several miles from the Turkish coast, and for many migrants the quickest and easiest route into the EU.
Greek Prime Minister Alexis Tsipras has said Greece cannot cope with the influx of arrivals, and that his country’s economic problems mean it is facing a humanitarian “crisis within a crisis”.