"There have already been 12 deaths," Krzysztof Mejer, a spokesman for the regional administration, told journalists at the scene.
According to police at least 80 people had already been hospitalised and another 100 were still believed to be trapped under the corrugated metal roof at 9:00 pm (2000 GMT).
Rescuers with sniffer dogs and lifting equipment were working to reach the survivors under the jumble of metal beams and sheets, with ambulances at the ready to rush the injured to hospital.
Voices of the injured could still be heard, with one rescuer heard screaming "Bring me another four body bags!"
At least one women trapped in the debris had called firefighters on her cell phone, saying she was surrounded by many injured people, police said.
Earlier firefighters said that as many as 500 people may have been inside the hall when the roof caved in during a carrier pigeon exposition in Chorzow,a suburb of the southern industrial city of Katowice.
"The snow accumulated on the roof of the hall could have been the cause of the accident," local fire brigade spokesman Janusz Jonczyk said. The region had recently received more than 30 centimeters of snow.
The accident comes less than four weeks after 15 people were killed in neighbouring Germany when the roof of a skating rink collapsed under the weight of snow.
"The roof was made of corrugated iron, more than 10 meters (30 feet) high; several hundred square meters caved in," Katowice police spokesman Andrzej Gaska said.
A correspondent for TVN television said the roof collapsed in the middle at
5:15 pm. Another part of the roof caved in shortly before 7:00 pm leaving a tangled mass of metal covered by snow.
"There was a terrible noise then the roof collapsed," said Jan Panek, a miner in his 50s who received an injury to his knee. "I was sitting about two meters from the roof which collapsed. It all happened really quickly."
Mr Gaska said the rescue operation could take all night as temperatures in the region plunged to minus 13 degrees Celsius and work was complicated by the snow.
Mr Panek, who was being treated a Katowice hospital, said luckily the crowd had thinned out just before the accident.
"If the roof had collapsed an hour earlier then there would have been a massacre," he said. "The exposition hall was so packed with people you couldn't get by people."
Several foreigners were among the injured. "Among the injured, there were definitely two Germans, a Czech, a Belgian, and a Slovak," said a staffer at the crisis center set up by the regional administration in the southern city of Katowice. The nationalities of the dead had not yet been determined.
Rescue teams with dogs had been dispatched by helicopter from all over the country.
Prime Minister Kazimierz Marcinkiewicz, vactioning at a nearby ski resort, was expected to visit the scene of the accident, where nervous people missing family members had already gathered.
"Have you seen Tomek Michalski, with a beard? He works here!" the man's teary-eyed mother asked every rescue workers who passed.
Keeping carrier pigeons is a popular pastime in southern Poland's Silesia region. Mr Mejer said the exposition had drawn many families as well as miners from the region.