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, with fears that the toll could eventually run into the hundreds.

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

But hundreds more may be missing, judging from the testimony from survivors and the fact that no women and children have yet been rescued.

"This could be the worst maritime tragedy in Greece in recent years," Stella Nanou of the UNHCR refugee agency told state broadcaster ERT.

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

"It's really horrific," UNHCR staffer Erasmia Roumana told AFP at the port of Kalamata.

The survivors were "in a very bad psychological situation", she added.

"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 lifejackets either on (the migrants) or the boat," local rescuer Constantinos Vlachonikolos told Proto Programma radio.

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

Two patrol boats, a navy frigate, three helicopters and nine other ships were searching the waters west of the Peloponnese peninsula, one of the deepest parts of the Mediterranean, a coastguard spokeswoman told AFP.

'I need my mother'

In a telegram, Pope Francis offered "heartfelt prayers for the many migrants who have died, their loved ones and all those traumatised by this tragedy".

Greece has declared three days of mourning over the tragedy and a senior prosecutor has been assigned to investigate.

Political parties suspended their campaigns for June 25 national elections.

"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.

He told ERT that a common tactic used by smugglers was to "lock people up to maintain control".

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 that the boat might have capsised 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, who spoke to survivors at the port, 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.

Authorities said the migrants had departed from Libya and were heading for Italy.

The survivors, mainly from Syria, Egypt and Pakistan, are for the moment being housed in a port warehouse. They will be interviewed by Greek officials, who are looking for possible smugglers among them.

Acting migration minister Daniel Esdras told ERT that the survivors would 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 said.

Along with Italy and Spain, Greece has been one of 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 overall was in April 2015, when between 800 and 900 migrants died on a trawler that sank within sight of a Portuguese rescue freighter.