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.

Though 78 bodies had been recovered and 104 people saved from the sea so far, hundreds more may be missing as no women and children have yet been rescued.

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

"This could be the worst maritime tragedy in Greece in recent years," Stella Nanou of the United Nations High Commissioner for Refugees (UNHCR) said.

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 the June 25 national elections.

Greece and other Mediterranean countries have been at the forefront of the influx of migrants since 2015. The often deadly trips across the Mediterranean have so far claimed the lives of thousands, according to UNHCR.

Human traffickers arrested, EU migration policies questioned

Nine Egyptian nationals suspected of human trafficking were arrested on Thursday by the Hellenic Coast Guard in connection with the deadly shipwreck. They were among the survivors of the tragedy.

Thousands of protesters rallied in Athens and the northern city of Thessaloniki on Thursday evening demanding European Union (EU) migration policies be relaxed.

In Kalamata, protesters marched outside the migrants' shelter. "Crocodile tears! No to the EU's pact on migration," read one banner.

EU migration policy "turns the Mediterranean, our seas, into watery graves," leftist Greek leader Alexis Tsipras who served as prime minister during the peak of Europe's migration crisis, said during a visit in Kalamata on Thursday.

Under a conservative government in power until last month, Greece took a harder stance on migration, building walled camps and boosting border controls.

Greece's government spokesman Ilias Siakantaris said that the biggest challenge for EU border states"is forging a comprehensive EU solution on migration and asylum that respects international law and inclusive humanism."

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(With input from agencies)