"We must take steps to ensure that such accidents are avoided in future. Measures to that end have already been taken: we are very determined," he said in a televised speech, in which he also said he would supervise the probe into the causes of the disaster.

Poland declared on Sunday three days of mourning after the catastrophe, probably the result of heavy snow piling on the roof. Two children were reported to have died.

At least nine foreigners -- two Czechs, two Germans, two Slovaks, a Belgian, a Dutchman, and a Romanian -- are believed to be among the dead. Local authorities said 18 bodies had not been identified and there were likely to be foreigners among them.

At least 200 people were attending the racing pigeon exhibition when the currogated metal roof collapsed.

More than 140 people were injured, including dozens of foreigners, according to various sources.

Late Sunday, 76 of the 141 people who were injured in the accident were still in hospital, Poland's PAP news agency said.

Thirteen of them were believed to be foreigners, Krzysztof Mejer, a spokesman for governor's office in Silesia, was cited by PAP news agency as saying.

Rescuers abandoned the search for survivors late Sunday morning after temperatures plunged to minus 17 degrees Celsius (one degree Fahrenheit) 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.

The exhibition hall roof was covered in deep, heavy snow, a government official said Sunday, contradicting reports by the building's manager that the roof had been cleared of snow.

"The parts of the roof that collapsed were covered with a layer of frozen snow that was 50 centimetres (1.6 feet) deep," Transport and Buildings Minister Jerzy Polaczek told a press conference in Chorzow, making public the preliminary findings of a probe into the disaster.

An AFP reporter at the scene said he had seen a metre (three feet) of ice on the pieces of roof which had hit the ground.

But Grzegorz Slyszyk, a spokesman for the company that managed the building, insisted accumulated snow had not caused the disaster.

Some trapped victims had called loved ones on their mobile phones from the ruins of the hall in this southern town, 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 late Saturday from under the rubble, hours after disaster struck the hall in Chorzow on the outskirts of the city of Katowice.

He was one of the last survivors to be pulled from the wreckage around 10:00 pm (2100 GMT) on Saturday night, and medical experts helping with the rescue effort 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 snow-covered metal roof had acted like a freezer.

Specialised mining teams, used to operating in pitch black, had worked alongside police specialists and sniffer dogs in the desperate search for the living.

Police said the hall could hold 700 people but the crowds had dwindled before the disaster happened.

Kaczynski declared three days of national mourning starting Sunday afternoon. "It is the biggest catastrophe in democratic Poland," he said.

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.

The government also ordered local authorities to clear snow from all buildings open to the public.

Among the survivors seen on Sunday were the pigeons, their plumage matted and ragged. Rescue workers slipping on the icy metal sheeting carried the birds out in the cages which, along with their feathers, had saved their lives.

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