Millions of people are facing travel disruption and increased Covid restrictions over Christmas, as the surging Omicron variant sees flights cancelled and safety curbs tightened.
Italy, Spain and Greece have made face masks compulsory outdoors again.
Catalonia, in northern Spain, has imposed an overnight curfew, and the Netherlands is in a strict lockdown.
Despite early findings that Omicron is milder than other variants, scientists are concerned by the number of cases.
Record infections were tallied in the UK, France and Italy on Thursday.
In the US, daily Omicron cases have risen beyond the peak of the recent Delta wave, and hospitals are filling up across the country.
"When we have millions and millions and millions of people, all sick, all together at one time, it doesn't take a large percentage of those people to topple over the hospitals," Dr Hallie Prescott, associate professor of internal medicine at the University of Michigan, told the New York Times.
America's top infectious disease expert, Dr Anthony Fauci, warned earlier this week that Christmas travel would increase the spread of the variant even among the fully vaccinated.
Thousands of flights have been cancelled across the world, according to the FlightAware website.
On Christmas Eve (Friday), US airlines said they were already suffering from staffing shortages due to flight crews testing positive or being forced to self-isolate.
United Airlines said rising numbers of Omicron cases had "had a direct impact on our flight crews and the people who run our operation", adding that it was contacting impacted passengers in advance of them coming to the airport.
The US is to lift travel restrictions imposed on eight African countries because of concerns about the Omicron variant on 31 December, the White House confirmed.
Travellers from South Africa, Botswana, Zimbabwe, Namibia, Lesotho, Eswatini, Mozambique and Malawi had been blocked since 29 November.
In Australia, too, thousands of festive journeys were affected on Friday with more than 100 domestic flights from Sydney and Melbourne to other cities cancelled.
A spokesperson for Jetstar, which accounted for many of the cancellations, said the airline had rebooked "the vast majority" of affected passengers "within a few hours of their original departure time so they can get to their destination in time for Christmas".
Despite the upheaval, many Australians may be celebrating the fact they can travel between states over the holidays for the first time in two years.