The collapse happened in the southern town of Chorzow.
Rescuers abandoned the search for survivors late on Sunday morning after temperatures plunged to minus 17 degrees Celsius overnight and cries emerging from the twisted sheet-metal wreckage died into silence.
"The chances of finding a survivor are close to nought," national fire chief Kazimierz Krzowski said.
"The rescue phase of the operation is over.”
Medical experts helping with the rescue said it was unlikely anyone could have survived overnight beneath the frozen metal.
Marek Brodzki, a surgeon in charge of an 18-person medical team at the site, said the metal of the building's remains, covered in snow, had acted like a freezer.
"The rescuers are cutting into the sheet metal, boring holes into it. But inside it's even colder," he said.
Specialised mining teams, used to operating in pitch black, had worked alongside police specialists and sniffer dogs in the desperate search for the living.
Final calls
Some trapped victims had called loved ones on their mobile phones from the ruins of the hall, describing the frozen corpses around them and the metal sheets that boxed them in during their last moments alive.
One victim, Tomek Michalski, called his mother in tears from under the rubble, hours after disaster struck the hall.
"Both of his legs and his shoulder are blocked by metal bars," she said.
"Next to him, a young woman is dead. He tried to save her life. She was a colleague. She had a six-month-old boy."
Tomek Michalski was one of the last survivors to be pulled from the wreckage around 10:00 pm (2100 GMT) on Saturday night.
At least 200 people were attending the racing pigeon exhibition when the roof fell.
Police said the hall could hold 700 people but the crowds had dwindled before the disaster happened.
More than 140 people were injured, including 100 Belgians and dozens of other foreigners.
"We heard a terrible noise and then the whole roof collapsed," 60-year-old Belgian vet Henk Weerde said, as rescue workers led him to an ambulance, his head covered in blood.
Foreigners dead
At least eight foreigners were among the dead, from Belgium, the Czech Republic, Germany, the Netherlands, Romania and Slovakia, police and relatives said.
A Dutchman who was among exhibitors at the show told news agency AFP that his father, Dick had died in the disaster, "He is dead," Frederic Basch said.
"Before I was able to escape, I saw his body. I have also had confirmation from the consulate."
Frederic Basch and his father had travelled to Chorzow from Deil in the Netherlands, and had a stand exhibiting pigeon food at the show.
They were among some 20 Dutch racing pigeon enthusiasts at the exhibition.
"Our Polish representative died with my father," Mr Basch added.
The German foreign ministry said that two Germans -- a 65-year-old man and a 35-year-old woman -- were killed in the disaster, and another four were injured.
The consul general of the Czech Republic, Josep Byrtus, told AFP that two Czech nationals died and Slovakia has reported that two of its nationals were still missing.
The bodies of six victims still needed to be identified.
"A friend of mine lost almost his whole family: his wife, daughter, his grandson. His son-in-law and son are still missing. My friend is sorry he got out alive," a policeman said.
Priests were also on hand in this overwhelmingly Catholic country, to administer the last rites.
A rescue team with a specially trained sniffer dog will search the rubble for more bodies Monday.
Nation in mourning
President Lech Kaczynski declared three days of national mourning starting on Sunday afternoon.
"It is the biggest catastrophe in democratic Poland," he said.
"The toll which is not expected to change is 66 dead including two children," President Kaczynski said, adding that 15 of the injured were still in critical condition.
The presidents of France, Russia and Slovakia as well as Germany's chancellor offered their condolences in separate statements.
Panic
Survivors say it all happened quickly, on the second day of the exhibition for racing pigeon enthusiasts, the fourth biggest such show in Europe.
"We heard a terrible noise, and then the roof caved in," said Henk Weerde, a 60-year-old Belgian veterinarian.
"I was sitting two meters from the bit of roof that collapsed. It was all so quick, probably three seconds," Jan Panek, a Polish visitor to the show, told AFP by phone from his hospital bed.
"If the roof had collapsed an hour earlier, it would have been a massacre. The exhibition hall was packed then. There were so many people in there, you couldn't even move," he added.
"We broke open an emergency exit. There was utter panic. We fled like rats," said Joseph de Scheemaecker, a Belgian exhibitor at the show.
"I had fallen asleep on a chair and was woken up by screams. The lights were all out. I saw a glimmer in the distance, like in a horror film. I went towards it," said Polish man Ryszard Kruszczynski, who lost two friends in the disaster.
Investigation
Prime Minister Kazimierz Marcinkiewicz, who interrupted a skiing holiday to visit the scene, ordered an investigation amid suggestions that too much snow had been allowed to accumulate on the five-year-old structure.
There are also reports that some people were killed because emergency exits were locked.
The government also ordered local authorities to clear snow from all buildings open to the public.
An AFP reporter at the scene said he had seen a metre of ice on the pieces of roof which had hit the ground.
Interior Minister Jerzy Polaczek said the layer was half that thick at 50 centimetres.
Chorzow fire brigade spokesman Janusz Jonczyk said heavy snow on the roof appeared to have caused the collapse.
But many locals said there’d been no significant snowfall since December.
A spokesman for the building's management company said snowfall was regularly removed.