Survivors from a fishing boat that sank off southern Greece in one of Europe's worst migrant disasters say up to 100 children may have been on board.
At least 78 people have already been confirmed dead in the disaster.
But many more could still be missing at sea, with reports suggesting that up to 750 people were aboard the vessel.
At least 11 arrests have been made including several Egyptians on suspicion of people trafficking, Greek TV reports.
The coastguard has been criticised for not intervening earlier but authorities say their offers of aid were refused.
Rescuers are still searching the area where the boat capsized almost 50 nautical miles off the south-west coast, as hopes of finding more survivors dwindle.
The boat had been heading to Italy from the Libyan port of Tobruk when it went down.
Images showed the decks packed with people, but accounts of a large number of women and children in the hold of the ship have come from medics who treated the mostly male survivors.
The senior doctor at Kalamata General Hospital told the BBC as many as 100 children were on the vessel.
"[The survivors] told us there were children in the bottom of the ship. Children and women," said Dr Manolis Makaris, head of cardiology.
He said two patients had given him estimated figures.
"One told me about 100 children, the other about 50, so I don't know the truth - but it is many," he added.
Dr Makaris said he believed as many as 600 people could have died in the disaster.
"The exact number of all the people who were on the boat was 750. This is the exact number that everyone told me about this," he said.
Families of some Egyptian children who were missing had sent him photographs of their young relatives, he said, in the hope he would recognise them after treating them.
"It was a tragedy," he said. "Everyone in Europe must not accept this situation. We have to do something. Everyone has to do something so it doesn't happen again."
A reporter from Greece's ANT1 channel asked a survivor if there were 100 children on board, to which the survivor replied: "Yes."
The BBC has not been able to independently verify the figure, but it was corroborate by the charity Save the Children, citing testimonies from survivors.
Families of some of the missing have arrived in Kalamata in search of their loved ones.
"My relatives were on the boat," said Aftab, who had travelled from the UK and said at least four of his relatives from Pakistan were unaccounted for.
"We've had confirmation. We found one of the relatives in [the rescue centre]. But the others we haven't got hold of yet," he told the BBC.
A Syrian man from the Netherlands broke down as he revealed his wife and brother-in-law were missing.
Activist Nawal Soufi was the first to raise the alarm after being contacted by people on the boat on Tuesday morning.
The coastguard said initial contact was made with the fishing boat at 14:00 local time (11:00 GMT) on Tuesday, and no request for help had been made.
It said the Greek shipping ministry had made repeated contact with the boat and was told it simply wanted to sail on to Italy.
Two commercial ships provided water during the evening. In a Facebook post, Ms Soufi said the situation became "complicated" when a ship approached the vessel and tied ropes to it while throwing bottles of water on board.
She said some of the people on board had felt in "extreme danger" due to fears the rope could cause the boat to flip, and that fights on board over water could result in it capsizing. The boat then moved away.
The coastguard said that in the early hours of Wednesday the boat's engine broke down and that people on board started to move around causing it to capsize. All of the 104 people rescued were male.
Alarm Phone, an emergency helpline for migrants in trouble at sea, complained that the coastguard had been "aware of the ship being in distress for hours before any help was sent", adding that authorities "had been informed by different sources" that the boat was in trouble.
However, coastguard spokesperson Nikos Alexiou said they tried to convince the vessel to get help and "stayed by if they needed us to save people".
Former Greek Prime Minister Alexis Tsipras visited Kalamata on Thursday and spoke to survivors about what they thought had gone wrong.
"The Greek coastguard asked the vessel to follow them, but they couldn't," a translator explained. "The coastguard then threw a rope but because they didn't know how to pull the rope, the vessel started dangling right and left,"
"The coastguard boat was going too fast but the vessel was already dangling to the left, and that's how it sank."
Greece is observing three days of mourning. Campaigning has been suspended ahead of the parliamentary election on 25 June and a TV debate due to happen on Thursday has been cancelled.
The country is one of the main routes into the European Union for refugees and migrants from the Middle East, Asia and Africa.
Last month the Greek government came under international criticism over video reportedly showed the forceful expulsion of migrants who were set adrift at sea.
At least 78 people have already been confirmed dead in the disaster.
But many more could still be missing at sea, with reports suggesting that up to 750 people were aboard the vessel.
At least 11 arrests have been made including several Egyptians on suspicion of people trafficking, Greek TV reports.
The coastguard has been criticised for not intervening earlier but authorities say their offers of aid were refused.
Rescuers are still searching the area where the boat capsized almost 50 nautical miles off the south-west coast, as hopes of finding more survivors dwindle.
The boat had been heading to Italy from the Libyan port of Tobruk when it went down.
Images showed the decks packed with people, but accounts of a large number of women and children in the hold of the ship have come from medics who treated the mostly male survivors.
The senior doctor at Kalamata General Hospital told the BBC as many as 100 children were on the vessel.
"[The survivors] told us there were children in the bottom of the ship. Children and women," said Dr Manolis Makaris, head of cardiology.
He said two patients had given him estimated figures.
"One told me about 100 children, the other about 50, so I don't know the truth - but it is many," he added.
Dr Makaris said he believed as many as 600 people could have died in the disaster.
"The exact number of all the people who were on the boat was 750. This is the exact number that everyone told me about this," he said.
Families of some Egyptian children who were missing had sent him photographs of their young relatives, he said, in the hope he would recognise them after treating them.
"It was a tragedy," he said. "Everyone in Europe must not accept this situation. We have to do something. Everyone has to do something so it doesn't happen again."
A reporter from Greece's ANT1 channel asked a survivor if there were 100 children on board, to which the survivor replied: "Yes."
The BBC has not been able to independently verify the figure, but it was corroborate by the charity Save the Children, citing testimonies from survivors.
Families of some of the missing have arrived in Kalamata in search of their loved ones.
"My relatives were on the boat," said Aftab, who had travelled from the UK and said at least four of his relatives from Pakistan were unaccounted for.
"We've had confirmation. We found one of the relatives in [the rescue centre]. But the others we haven't got hold of yet," he told the BBC.
A Syrian man from the Netherlands broke down as he revealed his wife and brother-in-law were missing.
Activist Nawal Soufi was the first to raise the alarm after being contacted by people on the boat on Tuesday morning.
The coastguard said initial contact was made with the fishing boat at 14:00 local time (11:00 GMT) on Tuesday, and no request for help had been made.
It said the Greek shipping ministry had made repeated contact with the boat and was told it simply wanted to sail on to Italy.
Two commercial ships provided water during the evening. In a Facebook post, Ms Soufi said the situation became "complicated" when a ship approached the vessel and tied ropes to it while throwing bottles of water on board.
She said some of the people on board had felt in "extreme danger" due to fears the rope could cause the boat to flip, and that fights on board over water could result in it capsizing. The boat then moved away.
The coastguard said that in the early hours of Wednesday the boat's engine broke down and that people on board started to move around causing it to capsize. All of the 104 people rescued were male.
Alarm Phone, an emergency helpline for migrants in trouble at sea, complained that the coastguard had been "aware of the ship being in distress for hours before any help was sent", adding that authorities "had been informed by different sources" that the boat was in trouble.
However, coastguard spokesperson Nikos Alexiou said they tried to convince the vessel to get help and "stayed by if they needed us to save people".
Former Greek Prime Minister Alexis Tsipras visited Kalamata on Thursday and spoke to survivors about what they thought had gone wrong.
"The Greek coastguard asked the vessel to follow them, but they couldn't," a translator explained. "The coastguard then threw a rope but because they didn't know how to pull the rope, the vessel started dangling right and left,"
"The coastguard boat was going too fast but the vessel was already dangling to the left, and that's how it sank."
Greece is observing three days of mourning. Campaigning has been suspended ahead of the parliamentary election on 25 June and a TV debate due to happen on Thursday has been cancelled.
The country is one of the main routes into the European Union for refugees and migrants from the Middle East, Asia and Africa.
Last month the Greek government came under international criticism over video reportedly showed the forceful expulsion of migrants who were set adrift at sea.
Greece boat disaster: Capsized boat had 100 children in hold, BBC told
Survivors from a fishing boat that sank off Greece say as many as 750 people may have been on board.
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