ROME - Bitterly cold weather gripped parts of Europe causing travel chaos on Monday, and a rare snow storm in Rome prompted Italian authorities to call in the army to help clear the streets.

A Siberian weather system British forecasters dubbed the "beast from the east" brought snow, strong winds and the coldest temperatures for years to many regions.

The freeze was expected to continue for much of the week, and the World Meteorological Organization said daily minimum temperatures below 0 C were expected even in southern Europe.

"This cold period ... could be a risk to life for vulnerable people exposed to the cold," it said.

Piazza Venezia, Rome's central square which is usually a cacophony of car horns and a tangle of traffic, was eerily empty, quiet and white as dawn broke.

In St. Peter's Square, priests and seminarians from the Vatican threw snowballs at each other. Near the Colosseum, students skied down the Oppian Hill.

It was the heaviest snowfall in Rome in six years and the largest for the end of February in decades. The city, which is not equipped to deal with snow emergencies due to their rarity, asked other areas to send in snow ploughs to help clear roads.

Schools were ordered shut and many people could not reach their work. Police asked residents to stay at home if possible.

Rome's Fiumicino airport was forced to operate with only one runway during the night, when 10 cm (4 inches) of snow fell in less than four hours. The capital's second airport, Ciampino, was closed overnight and workers were clearing its two runways.

Italian aviation officials said Ryanair had cancelled all flights to and from Ciampino, its Italian hub. Most of Italy's other main airports were open, albeit with flight delays.