Greek rescuers on 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 on 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 travelled 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."
'I need my mother'
"One young man started to cry and said, I need my mother... This voice is inside my ears. And will always be inside," Red Cross nurse Ekaterini Tsata told AFP.
Around 30 people were hospitalised with pneumonia, dehydration and exhaustion but are not in immediate danger, officials said.
Some of those rescued are under 18.
"The fishing boat was 25-30 metres long. Its deck was full of people, and we assume the interior was just as full," coastguard spokesman Nikolaos Alexiou told ERT.
Government spokesman Ilias Siakantaris on Wednesday said there were unconfirmed reports that up to 750 people had been on the boat.
In a telegram, Pope Francis offered "heartfelt prayers for the many migrants who have died, their loved ones and all those traumatised".
But Doctors Without Borders (MSF) said the disaster was entirely preventable.
"What happened is the consequence of the absence of safe and legal pathways to come to Europe," said Juan Matias Gil, of MSF Sea.
Police said thousands of people in Athens and the northern city of Thessaloniki protested with signs reading "the government and the European Union kill" and "No to fortress Europe, solidarity with refugees".
Questions over rescue
The coastguard said a surveillance plane with Europe's Frontex agency had spotted the boat on Tuesday afternoon, but that the passengers had "refused any help".
The boat's engine gave up shortly before 2300 GMT on Tuesday and the vessel later capsized, Siakantaris said, sinking in around 10 to 15 minutes.
Alexiou, the coastguard spokesman, suggested the boat might have capsized earlier if the coastguard had attempted to intervene.
"You cannot divert a boat with so many people on board by force unless there is cooperation," he said.
It was "fortunate" that rescue ships were nearby or more lives would have been lost, he added.
But leftist former prime minister Alexis Tsipras said they had "called for help".
"What sort of protocol does not call for the rescue... of an overloaded boat about to sink?" he asked.
The head of Frontex, Hans Leijtens, arrived in Greece on Thursday "to better understand what happened since Frontex played a part" and show "solidarity and help to Greek colleagues, who did everything possible to save lives", he said.
Migrants headed for Italy
Officials say the migrants had departed from Libya towards Italy.
The survivors, mainly from Syria, Egypt and Pakistan, are being housed in a Kalamata warehouse.
Acting migration minister Daniel Esdras said the survivors would eventually be taken to Malakasa migrant camp near Athens by Friday. Greece would examine their asylum claims, but those not entitled to protection would be sent home, he added.
Greece, Italy and Spain are among the main landing points for tens of thousands of people seeking to reach Europe from Africa and the Middle East.
The worst migrant tragedy in Greece was in June 2016, when at least 320 people were listed as dead or missing in a sinking near Crete, according to AFP records going back to 1993.
The Mediterranean's worst disaster was in April 2015, when between 800 and 900 migrants died on a trawler that sank within sight of a Portuguese rescue freighter.
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