Greek rescuers Thursday scoured the Ionian Sea for survivors a day after a fishing boat overloaded with migrants capsized and sank, killing at least 78 people, as police arrested nine suspected people smugglers.

Fears were growing that the final toll could eventually run into the hundreds.

"We are witnessing one of the biggest tragedies in the Mediterranean and the numbers announced by the authorities are devastating," the UN International Organization for Migration said.

As frantic relatives in the migrants' home countries sought details of loved ones, the coastguard said 78 bodies had been recovered and 104 people saved from the sea so far.

Police Thursday arrested nine Egyptians on suspicion of people smuggling -- one of them the captain of the boat carrying the migrants.

They were detained at the port of Kalamata, where the survivors are being cared for, said Greek news agency ANA.

A survivor told hospital doctors in Kalamata he had seen a hundred children in the boat's hold, broadcaster ERT reported.

"It's really horrific," UN refugee agency staffer Erasmia Roumana told AFP. The survivors were "in a very bad psychological situation".

"Many are under shock, they are so overwhelmed," she said. "Many of them worry about the people they traveled with, families or friends."

Photographs handed out by the coastguard showed a rusty blue boat with scores of people crammed on deck.

"It was like an abandoned ship... we saw no lifesavers or life jackets either on (the migrants) or the boat," local rescuer Constantinos Vlachonikolos told Proto Programma radio.

"We've never seen anything like this before."