Refugees on the Greek island of Kos were attacked in the early hours of Friday by "thugs" with bats, telling them to "go back to their countries," rights group Amnesty International said.
Amnesty staff witnessed "a violent attack on refugees" by a group of between 15 and 25 people on the island, the group said in a statement, adding that riot police had used tear gas to disperse the crowd.
Local police denied they had fired tear gas but said pepper spray had been used earlier that night to break up a crowd of around 1,000 people who gathered outside the police station after a row broke out between Syrian and Iraqi migrants. The migrants had thrown stones at officers, a police source said.
Kos is one of several Greek islands struggling to cope as thousands of people arrive from Turkey in flimsy boats, most of them refugees fleeing war-torn Syria.
The numbers arriving on the islands have soared in recent weeks as the migrants – many of them paying people smugglers $1,000 or more for the risky crossing – seek to take advantage of the calm summer weather.
The islands' infrastructure has strained under the weight of the arrivals and Amnesty called for "immediate action to end the prolonged suffering of thousands of refugees, including many children, staying in inhumane conditions."
The rights group said babies as young as a week old were among the crowds waiting for days in scorching heat to be registered before they can travel on to the Greek mainland, from where many hope to head north to other countries in Europe.