Four skiers have been killed and another injured in an avalanche in the French Alps, as the deadly Siberian weather system nicknamed "Beast from the East" continued to cause chaos across Europe.

Key points: More than 20 people have died in Poland since the cold snap began

More than 20 people have died in Poland since the cold snap began Emergency shelters have been opened in many countries to help the homeless and citizens left stranded

Emergency shelters have been opened in many countries to help the homeless and citizens left stranded Hundreds of flights were cancelled in the United Kingdom, Ireland and Switzerland

The group was on a ski touring expedition in an off-trail area near Entraunes, close to the Italian border in the southern Alps.

The identities of the skiers were not immediately released. Prosecutor Jean-Michel Pretre of Nice said in remarks shown on French television that they were "seniors", all with experience in backcountry skiing.

Meanwhile, the extraordinary weather system, which has now claimed more than 50 lives, continued to bring disruption to many countries across Europe.

More than 20 people have died in Poland alone since the cold snap began, many believed to be rough sleepers.

Some Polish cities have installed heaters in the streets in a bid to help the most vulnerable.

Emergency shelters have been opened in many countries to help the homeless and citizens left stranded by disrupted transport services.

People were able to walk and skate on Amsterdam's Prinsengracht canal after it froze. ( AP: Mike Corder )

The big chill also froze canals in the Dutch capital, Amsterdam. Ice on the historic Prinsengracht canal was thick enough for residents to lace up their skates and glide across its frozen surface. Tourists without skates slid across the ice, taking selfies.

"It's just cool. You can go fast and you see the world from a slightly different perspective," said skater Noldus Reijnders.

Hundreds of flights were cancelled in the United Kingdom, Ireland and Switzerland. Trains broke down. Motorists found themselves stuck on highways and trapped in frosty conditions for hours.

"This is particularly unusual weather," British Transport Secretary Chris Grayling said.

"It's something that happens very rarely in this country."

Heavy snow in Dublin has caused commuter chaos. ( Reuters: Clodagh Kilcoyne )

Up to a metre of snow was reported in eastern Ireland, and travellers were stranded south and west of the capital, Dublin.

Heathrow Airport tweeted on Friday that it was working with airlines to consolidate the flight schedule "to provide more certainty around departing flights", amid the extreme winter conditions across the UK and Europe.

More than 350 flights were cancelled. Gatwick, London City, Edinburgh and Glasgow airports also reported cancellations.

Sorry, this video has expired Extreme weather batters Europe ( Yvonne Yong )

The army sent 20 troops and 10 four-wheel drive vehicles to Shropshire, the county south of Liverpool, and the Royal Marines sent resources to Devon and Cornwall on the south-west coast after police asked for help.

One train travelling from London's Waterloo Station to Weymouth ground to a halt outside New Milton, stranding motorists for hours. By mid-afternoon, South Western Railway and Southeastern had urged customers not to travel.

Some commuters reported that rail doors refused to open as the push-button mechanisms froze in the cold. Thousands of homes are without electricity as temperatures remain below freezing with bitter winds.

One police force in Scotland tweeted a picture of a patrol car beside a snowdrift almost as high as the vehicle itself to show drivers why they should stay home. "PLEASE AVOID THIS AREA," the post said.

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Commuter chaos in England

In central England, volunteers brought hot drinks and blankets to stranded drivers as they waited for help. Eleanor Kelly, 19, said those stranded in the Milnrow suburb of Rochdale included a father with a baby and toddler in his car.

"We've been trying to get to as many people as we can," she said.

Commuter Philip Brown endured more than 15 hours on a train travelling from London Waterloo to Bournemouth, Dorset. The average time for the journey is about two hours.

"I didn't have any food or water. There were no buffet facilities on board. The train lost power and we lost heating and lights," Mr Brown said.

"I couldn't tell you how cold it was, but it was cold enough to prevent you from sleeping … people were wrapping jumpers round their legs trying to keep warm."

But in Amsterdam, nobody was complaining about the cold. Residents on skates glided past tourists who slithered across the ice for pictures. One woman took her dog for a walk along the frozen canal.

Still, despite measures taken by authorities to help the ice develop, there were still some holes and parts of the canal were not frozen at all.

Mr Reijnders was wearing a special red ice pick around his neck just in case.

"If you sink through the ice — and there are still a few dangerous places — you can pull yourself out," he said.

The unusual cold spell has been felt as far south as the Mediterranean, with meteorologists calling it the worst weather the continent has seen since 1991.

More freezing snowy weather was forecast as Storm Emma approaches from Portugal and France, with warnings of more treacherous weather across southern England and Ireland.

Heavy snowfall from the Arctic blast hit Saint Peter's Square at the Vatican. ( Reuters: Max Rossi )

AP/Reuters