More than half the US threatened with ice, snow, cold

Freezing rain has fallen in parts of Texas as a huge, days-long winter storm began a trek that threatened to bring snow, sleet, ice, bone-chilling temperatures and extensive power outages to about half the US population.

Forecasters warned that catastrophic damage, especially in areas pounded by ice, could rival that of a hurricane.

At least 182 million people on Friday were under watches or warnings for ice and snow, and more than 210 million were under cold weather advisories or warnings. In many places those overlapped.

Utility companies braced for power outages because ice-coated trees and power lines can keep falling long after a storm has passed.

Freezing rain slicked roads in Lubbock, Texas, in the afternoon as temperatures dropped.

After sliding into the South, the storm was expected to move into the Northeast, dumping about 30cm of snow from Washington through New York and Boston, the National Weather Service predicted.

Frigid air that spilled down from Canada prompted the cancellations of classes at schools throughout the Midwest. Wind chills as low as minus 40C meant that frostbite could set in within 10 minutes, making it too dangerous to walk to school or wait for the bus.

Despite the bitter cold, a protest over an immigration crackdown went on as planned in Minnesota, with thousands of people demonstrating in downtown Minneapolis.

Nationwide, nearly 5000 flights were delayed or cancelled on Friday, many of them in Dallas and Chicago, according to the flight tracking website FlightAware. About 2800 were called off for Saturday.

The federal government put nearly 30 search and rescue teams on standby. Officials had more than seven million meals, 600,000 blankets and 300 generators placed throughout the area the storm was expected to cross, according to the Federal Emergency Management Agency.

President Donald Trump said via social media that his administration was co-ordinating with state and local officials and "FEMA is fully prepared to respond."

Boston declared a cold emergency through the weekend, and Connecticut was working with neighbouring New York and Massachusetts in case travel restrictions are needed on major highways.