Four people — including two children — were killed and more than 100 were injured during a thunderstorm in Poland's Tatra Mountains on Thursday, Prime Minister Mateusz Morawiecki said.

A fifth person was killed by the storm in the West Tatras in neighbouring Slovakia, mountain rescue services reported.

Lightning hit a group of tourists on Giewont, a 1,894-metre mountain in Poland, after a sunny morning turned stormy, according to witnesses quoted on private broadcaster TVN24.

Tourists climbing Giewont often aim to end their trip at the summit, where a 15-metre iron cross was installed in 1901. The cross is believed to attract lightning.

"Nobody expected such a sudden storm to break out, and from our human point of view it was something which was impossible to predict," said Morawiecki after a meeting of an emergency council in the southern town of Zakopane.

At least five people, including two children, were killed. (Bartlomiej Jurecki/The Associated Press)

The Slovak rescue service said a Czech tourist fell hundreds of metres down the side of a mountain and was killed after lightning knocked him off Banikov peak.

Rescuers with the Polish Tatra emergency service, known as TOPR, said they believe the lightning probably hit some of the metal chains installed on Giewont peak to aid tourists in their climb.

Some of the injured were brought by helicopter to a hospital in the Polish mountain resort of Zakopane, and Krakow province governor Piotr Cwik told reporters that the death toll could certainly rise.

Morawiecki said some among the over 100 injured were in very serious condition with severe burns or head injuries, as they fell after the lightning strikes or were hit by falling rocks.

Rescuers were also looking for two spelunkers who got trapped in a cave when a tunnel flooded. On Thursday, one of their bodies was found. (Marek Podmokly/Agencja Gazeta/Reuters)

The mayor of Zakopane, Leszek Dorula, declared Friday a day of mourning, Polish news agency PAP reported.

The Tatras, part of the Carpathian mountain range, are the highest mountains in Poland and in Slovakia, and attract tourists from near and far with scenic lakes and peaks that soar to 2,655 metres.

Thursday's lightning strikes were the worst accident in the Tatras since August 1937, when lighting killed four people on Giewont.

Joanna Sieradzka, spokesperson for the Krakow Ambulance Service, said on TVN24 that five helicopters had been involved in the rescue effort.

Tourist Grzegorz Pyzel told TVN24 he was halfway up Giewont peak with his wife in clear weather when suddenly they heard thunder and thought it was a jet overhead.

"But soon lightning struck and we turned back," Pyzel said. "Suddenly it started pouring, and you could hear thunder roaring from every possible direction."

The couple reached a shelter on Hala Kondratowa, at the foot of the mountain, and soon others started coming in, saying there were injured people farther up the mountain, he said.

In another rescue operation in the Tatra Mountains, emergency workers were searching for two spelunkers who went missing in a cave on Saturday after being trapped by rising water. Rescuers used small amounts of explosives to widen passages in uncharted parts of the Wielka Sniezna cave, Poland's deepest and longest, to look for the missing cavers.

On Thursday, the body of one of the two was found, an official said.