At least 4,000 migrants were evacuated from the Moria camp on the Greek island of Lesbos after a large fire destroyed tents and prefabricated homes, the media reported.
No injuries have been reported and some migrants have now been allowed to return to the camp, BBC reported.
Police are still investigating whether the fire was deliberate, but an aid worker says it started after a food dispute.
There are some 5,600 refugees currently in Lesbos, according to the UN, but the island only has capacity for 3,600.
The fire destroyed 30 per cent of the camp, according to Aris Vlashopoulos, an aid worker with the Swiss charity SAO.
"People are returning to the camp now as I can see. But the biggest number of the refugees are already on the streets, sleeping outside," he added.
According to Greek state news agency ANA, a brawl started on Monday after a rumour that refugees would be deported to Turkey en masse.
Many on the island are afraid of being returned to Turkey or their home countries, correspondents say.
The situation on Lesbos is exacerbated as many refugees land there and are held on the island until their asylum claim is fully processed.
Those granted asylum are permitted to journey to the Greek mainland, while others are turned back.
The procedure is part of the EU-Turkey deal brokered in March, in which the EU pledged to work towards visa-free travel for Turkish citizens to Europe’s Schengen zone in exchange for Ankara’s assistance in curbing migrants crossing to the Greek islands.