As many as 30,000 migrants could be killed attempting to cross the Mediterranean this year unless the crisis is brought under control, analysts have warned.
The death toll from capsizing disasters stands at 1,727 so far in 2015 - 30 times higher than the 56 fatalities by April 21 last year, according to the International Organisation for Migration (IOM).
It came as a European plan to tackle the emergency was criticised for not going far enough.
IOM said the total of 3,279 migrants deaths in the Mediterranean last year could be surpassed within weeks. Spokesman Joel Millman said that at the current rate the total for this year could reach 30,000.
The dire estimate came as the Office of the United Nations High Commissioner for Refugees (UNHCR) said the number killed in the shipwreck off the coast of Libya on Saturday is now believed to be more than 800.
Spokesman Arian Edwards said this made it "the deadliest incident in the Mediterranean we have ever recorded".
The UNHCR said there were around 850 people on board, including 350 Eritreans and people from Syria, Somalia, Sierra Leone, Mali, Senegal, Gambia, Ivory Coast and Ethiopia.
Only 28 people are known to have survived the tragedy and they arrived in Catania, Sicily, late last night. Children are believed to have been on the boat, but none have been found.
Carlotta Sami, of UNHCR, said migrants looked "exhausted, fragile, astonished to see so many people waiting for them" when they arrived in Catania.
Two alleged people smugglers, including one believed to have been the captain of the ship, have been detained by Italian authorities, according to reports.
Claims emerged today that the boat capsized after accidentally colliding with another ship which came to its aid.
Meanwhile Labour said EU proposals to ease the migrant crisis do not go "nearly far enough", with leader Ed Miliband warning it will be "a stain on the EU" if proper action is not taken.
EU leaders including David Cameron will meet in Brussels on Thursday to discuss the 10-point plan drawn up on Monday.
Proposals include strengthening the Triton patrolling service and seeking a military mandate to seize and destroy people-smugglers' boats, backed by increased budgets for Frontex, which manages the borders of the 28-nation bloc.
But Labour's shadow home secretary Yvette Cooper described the response as "painfully weak" and said the EU needed to restore a properly-funded search and rescue mission, after the Italian Mare Nostrum operation was halted last year.
Maurice Wren, chief executive of the Refugee Council, said the response to events in the Mediterranean was "inadequate" and Europe had to "raise its game".
He said: "The immediate issue is to ensure that people who do board rickety boats in Libya, setting sail for Europe, are given as much opportunity to reach safety, reach land safely. Europe should surely have no interest in simply refusing, turning its back on people in desperate need of help."
Former foreign secretary William Hague said there was no "quick fix" to the "absolutely catastrophic" loss of life.
He told the BBC Radio 4 Today programme there had been sound reasons for stopping the previous operation, which rescued people right up to the border of Libyan territorial waters, because more people were dying but he said the current operation was also not working.
His successor Philip Hammond said Mr Cameron will attend the summit willing to agree British support "for anything that will end this appalling situation in the Mediterranean but insistent that we link with a clear programme to tackle the problem upstream".
He said:"Tackle the traffickers, the criminal trafficking gangs, tackle the conditions in the countries of origin and countries of transit that are forcing people to get under the control of these traffickers and take some appalling risks."
On restarting the rescue, Mr Hammond said the Italians opted to end Mare Nostrum.
He went on: "One of the subjects that will be discussed at the European Council tomorrow is around the broader picture and how the EU supports the Italians, the Maltese and the Greeks, who are very much the countries on the frontlines here, to be able to provide adequate support to people who are caught up in this traffic.
"But we must not take our eyes off the principal challenge, which is to shut down the traffic in the first place.
"If people are falling off a cliff, you don't spend all your resources picking them up at the bottom of the cliff. You go and build a fence at the top of the cliff so they can't fall over it."
Save the Children said its staff in Italy have learned that 60 adolescents were on board the boat that sank off Libya.
Four boys who said they were under the age of 18 survived the journey, the charity said.
Spokeswoman Gemma Parkin said: " Children arriving off the boats are exhausted and traumatised from the ordeal they have been through, not just at sea but also during their long and dangerous land journeys.
"But they also tell us they are grateful to be alive and in a safe place - they know they are the lucky ones."
Mr Hammond later said search and rescue operations must be supported, telling Daily Politics on BBC Two he "absolutely" supported a " more formidable operation on the sea".
Deputy Prime Minister Nick Clegg said it was a mistake to abandon the search and rescue mission.
He told Channel 4 News: "I think with hindsight that was not the right decision, but even if you return to that old search and rescue system, that still doesn't provide you with a solution, a European solution, to a very real problem, which is thousands of people in wretched circumstances travelling huge distances, exiting ports in North Africa in the hands of illegal human smugglers and traffickers and perishing on the high seas in the Mediterranean."
Challenged that the European Union had decided on a policy of "let them drown", Mr Clegg said: "I'm afraid they were drowning whether you had a search and rescue operation or not.
"The solution to this is not going to be found at sea, this is the point. The solution to this can only be found by taking on the smugglers and the traffickers, can only be found by using our considerable aid and assistance budget to try and keep people in the communities where they are presently fleeing from."
He added: "Clearly what we should have done, with hindsight, is do more as the European Union to invest and make sure that the long arm of the law really reaches out to destroy the boats, to destroy the network of illegal human traffickers and smugglers, but also to make sure that we use the economic clout, the political clout, of the European Union collectively to try and stabilise those communities which at the moment are so wretched that people are taking their own life into their own hands."
Mr Clegg said it was a "ludicrous argument" to link the bombing campaign carried out by British and allied jets against Muammar Gaddafi's regime in Libya to the drownings.
"This Nigel Farage argument ... that with some extraordinary wisdom with hindsight we should have ignored the bended-knee pleadings of innocent men, women and children in Benghazi who were going to be killed by Gaddafi, that we should have known then that by helping them we were going to allow Eritreans and Somalis and others from other parts of Africa to go into the hands of illegal human traffickers ... it's a ludicrous argument."
In an account released by Save the Children, a teenage boy who survived the disaster said he fainted as the boat was about to capsize.
The 16-year-old from Somalia said he was placed on a rubber dinghy and taken to the bigger fishing vessel.
After around 800 people were loaded onboard the boat was so full that "we couldn't move", the boy's account claimed.
He said the migrants on board had no food or water and some were people were locked below.
At some point a call for help was issued by the boat and "when people saw the lights of the rescue boats everyone began to move to one side and the boat inclined to one side and then it turned over completely", his account said.
It added: "He then fainted and the next thing he woke up. "
The survivor said he is "too tired to think about his next move", Save the Children said.