With the big chill coinciding with the start of the big annual movement of people going home for Chinese New Year, travellers are having to brave both the unseasonal cold and the inconvenience of travel delays.

More than 2.9 billion passenger trips are expected to be made during "chunyun" - or "spring movement" - this year, but the unprecedented cold snap that has hit the country has resulted in train delays, flight cancellations and highway closures.

Pictures in media reports here showed passengers wearing thick layers of clothing as they thronged trains for the journey home at the start of the 40-day chunyun on Sunday, which saw 400,000 people depart from Beijing's train stations.

The cold snap, which saw the mercury in Beijing plunge to an unseasonably low minus 16 deg C over the weekend, has eased in northern China, but will continue to blight southern areas until today as the polar vortex responsible for the big chill moves south.

Up till yesterday night, temperatures in parts of southern China were set to drop to 3 to 6 deg C, while the lowest temperatures in the lower reaches of the Yangtze River could reach minus 10 deg C, according to the National Meteorological Centre (NMC).

Temperatures could reach or fall below historical lows in parts of western Sichuan province, and the coastal provinces of Zhejiang, Fujian and Guangdong.

These areas had already seen freak weather over the weekend, with rare snow in Chongqing and sleet in sub-tropical Guangzhou.

Officials in Zhejiang were quoted as saying that the cold had resulted in an economic loss of 380 million yuan (S$83 million) as the province suffered from record low temperatures of minus 20 deg C in the mountain areas.

The worst cold snap in years has led to at least four deaths so far, the official news agency Xinhua reported.

But the worst of the cold will likely be over by today.

The NMC said that in northern China, the mercury had already risen by between 8 deg C and 14 deg C from Saturday.