Rescue ships fanned out on Thursday in search of hundreds of refugees and migrants feared dead after their overcrowded fishing boat capsized and sank as they tried to reach Europe.

At least 79 people are confirmed to have died in the disaster.

Rescuers saved 104 passengers from the boat that sank in deep waters off Greece’s coast early on Wednesday while trying to travel from Libya to Italy.

Authorities fear that hundreds of others – including women and children – may have been trapped below deck.

If confirmed, that would make the tragedy one of the worst ever recorded in the central Mediterranean.

“The chances of finding [more survivors] are minimal,” retired Greek coast guard admiral Nikos Spanos told state-run ERT television.

The International Organization for Migration estimated that the vessel was carrying 700 to 750 people, including at least 40 children, based on interviews with survivors. Save the Children put that number higher, at about 100 children.

Reports suggested that the boat sank about 80km (50 miles) from the southern coastal town of Pylos.

Refugee activists, NGOs that rescue refugees, some European politicians and Pope Francis were among those who shared their sorrow and anger over the disaster.

The Vatican said the pontiff was “deeply dismayed” and offered “heartfelt prayers for the many migrants who have died, their loved ones and all those traumatised by this tragedy”.

‘Should be a wake-up call for EU governments’

Aerial pictures released by the Greek coast guard showed dozens of people on the boat’s upper and lower decks looking up, some with arms outstretched, hours before it sank.

Alarm Phone, which operates a trans-European network supporting rescue operations, said it received alerts from people on board a ship in distress off Greece late on Tuesday.

It said it had alerted Greek authorities and spoke to people on the vessel who appealed for help, and that the captain had fled on a small boat.

Government officials said that before capsizing and sinking around 2am on Wednesday, the vessel’s engine stopped and it began veering from side to side.

Of the survivors so far transferred to the port city of Kalamata, most were men, authorities said.

Al Jazeera’s John Psaropoulos, reporting from Kalamata, said doctors have treated several survivors, dozens of whom had symptoms of “near drowning”.

“They suffered from a type of pneumonia that occurs when the lungs are partly flooded with water,” he said. “Some have been sent back to a warehouse where others survivors have been staying.”

As Greece declared three days of mourning, the bodies of the victims were transferred to a cemetery near Athens for DNA tests. The search operation will continue for as long as needed, the coast guard said.

“[European Union] member states have gone to extraordinary lengths to close off all routes to children and their families seeking safety in Europe. Often their only option is to take dangerous journeys by boat,” said Daniel Gorevan, senior advocacy adviser at Save the Children.

“The fact that people continue to die in the Mediterranean should be a wake-up call for EU governments,” he warned.

Government sources said the chances of retrieving the sunken vessel, which had set off from the Libyan port of Tobruk, were remote. The area of international waters where the incident occurred is one of the Mediterranean’s deepest.

Independent refugee activist Nawal Soufi said in a Facebook post that she was contacted by people aboard the vessel in the early hours of Tuesday, and that she had been in contact with them until 11pm (20:00 GMT).

“The whole time they asked me what they should do and I kept telling them that Greek help would come. In this last call, the man I was talking to expressly told me: ‘I feel that this will be our last night alive,’” she wrote.

Greece is one of the main routes into the EU for refugees and migrants from the Middle East, Asia and Africa.

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

The country is currently governed by a caretaker administration pending an election on June 25.

Libya, which has had little stability or security since a NATO-backed uprising in 2011, is a major launching point for those seeking to reach Europe by sea, its people-smuggling networks mainly run by military factions that control coastal areas.

The United Nations has registered more than 20,000 deaths and disappearances in the central Mediterranean since 2014, making it the most dangerous migrant crossing in the world.