Commentators spoke of Britain being brought back to the 19th century, to the days before air travel. Aviation officials said that a complete shutdown of British airspace had not happened in living memory. Even after the Sept. 11, 2001, terrorist attacks, a no-fly zone was imposed only over central London but not the rest of the country.
The closure of British airspace was scheduled to last until at least 1 p.m. Friday.
"However, be aware that the situation cannot be said to be improving with any certainty," the National Air Traffic Services said late Thursday.
Copenhagen Airport said it would resume flights no earlier than noon Friday. Its closure affected Danes at the highest level: Denmark's Queen Margrethe II was to mark her 70th birthday Friday with a celebration attended by various crowned heads of Europe, but those guests' attendance was thrown into doubt.
A Copenhagen Airport spokesman said the airport was handing out blankets and water to stranded passengers.
"I don't know what's going to happen. My flight has been canceled, and I don't have a phone to call the airline with. I think I'll stay in the airport tonight; I think you can sleep here," 25-year-old Nick Shown of Maine told the Danish newspaper Politiken.
The disruption came just as airlines were beginning to see international passenger numbers rebound from a steep decline that began in late 2008. The direct economic effect of the disruption was hard to gauge because it was unclear Thursday how many cities would be closed to air traffic and for how long, said Steve Lott, a spokesman for the IATA.
But Lott said the closure of key international hubs, including London and Paris, could also hurt travel and tourism across Europe and in the United States.
"The bottom line is when you start closing or restricting European gateways, it has a ripple effect across the world," he said.
According to the U.S. Geological Survey, about 100 aircraft have had problems after encountering ash from previous eruptions, but there have been no fatalities.
Times staff writers Ann M. Simmons, Thomas H. Maugh II and Mitchell Landsberg in Los Angeles, Henry Chu in London and special correspondent Helen Hajjaj in Copenhagen contributed to this report.