UN refugee chiefs voiced fresh concerns over the viability of the controversial EU-Turkey agreement to deport thousands of illegal immigrants to Turkey after several migrants were hospitalised in riots and hundreds more broke out of a detention centre in Greece.

Three migrants were injured when police used stun grenades to break up a late-night protest on the island of Chios, as frustrations boiled over on the Greek islands where more than 5,000 are being detained ahead of the deportation scheme due to begin on Monday.

Rioting also broke out on Samos, leaving three migrants with stab wounds.

Later hundreds of angry migrants tore down part of the razor wire fence surrounding their holding centre on Chios and began walking to the port in protest, local police officials said.

In scenes reminiscent of the breakout of migrants from Keleti railway station in Budapest last September, video clips on Greek websites showed a column of 700 migrants and refugees, many of them women and children, walking along the tree-lined road to the port.