PARIS — Violent clashes erupted in Greek refugee camps among panicked migrants as Greece and the European Union pressed ahead on Friday with their intention to expel them from Europe and deport thousands back to Turkey, despite strong objections from rights groups and United Nations relief officials who say the plan is illegal and inhumane.

Hundreds of migrants broke out of an overcrowded detention center on the Greek island of Chios and began walking to the port to protest a European Union deal that went into effect in March, authorizing Greece to return them to Turkey if their applications for asylum in Europe were not accepted. The deportations officially begin on Monday.

Video clips in the Greek media showed migrants streaming away from the camp unhindered by the police, hours after a brawl broke out at the camp’s registration center. Several refugees were taken to a hospital after the riot police used stun grenades, and a help center run by Doctors Without Borders was destroyed, forcing the aid group to abandon its work. More than 1,500 migrants were being held at the center, designed for 1,200. Three people were also reported stabbed during a migrant riot on the island of Samos, where another detention facility operates.

Major humanitarian groups, including Doctors Without Borders, the United Nations refugee agency and the International Rescue Committee, recently suspended some work at detention centers to protest the European Union deal, which they say violates international law.