"The skies are totally empty over northern Europe," said Brian Flynn, deputy head of Eurocontrol, adding "there will be some significant disruption of European air traffic tomorrow."
The agency said about 16,000 of Europe's usual 28,000 daily flights were canceled Friday -- twice as many as were canceled a day earlier. Only about 120 trans-Atlantic flights reached European airports compared to 300 on a normal day, and about 60 flights between Asia and Europe were canceled.
The International Air Transport Association said the volcano was costing the industry at least $200 million a day.
Southern Iceland's Eyjafjallajokull glacier began erupting for the second time in a month on Wednesday, sending ash several miles (kilometers) into the air. Winds pushed the plume south and east across Britain, Ireland, Scandinavia and into the heart of Europe.
Gray ash settled in drifts near the glacier, swirling in the air and turning day into night. Authorities told people in the area with respiratory problems to stay indoors, and advised everyone to wear masks and protective goggles outside.
In major European cities, travel chaos reigned. Extra trains were put on in Amsterdam and lines to buy train tickets were so long that the rail company handed out free coffee.
Train operator Eurostar said it was carrying almost 50,000 passengers between London, Paris and Brussels. Thalys, a high-speed venture of the French, Belgian and German rail companies, was allowing passengers to buy tickets even if trains were fully booked.
London taxi company Addison Lee said it had received requests for journeys to cities as far away as Paris, Milan, Amsterdam and Zurich.
The disruptions hit tourists, business travelers and dignitaries alike.
German Chancellor Angela Merkel had to go to Portugal rather than Berlin as she flew home from a U.S. visit. Norwegian Prime Minister Jens Stoltenberg managed to get a flight to Madrid from New York but was still not sure when or how he would get back home.
The military also had to adjust. Five German soldiers wounded in Afghanistan were diverted to Turkey instead of Germany, while U.S. medical evacuations for troops in Iraq and Afghanistan are being flown directly from the warfronts to Washington rather than to a care facility in Germany. The U.S. military has also stopped using temporarily closed air bases in the U.K. and Germany.
Aviation experts said it was among the worst disruptions Europe has ever seen.
"We don't have many volcanoes in Europe," said David Learmount of Flight International, an aviation publication. "The wind was blowing in the wrong direction."
In Iceland, torrents of water carried away chunks of ice the size of small houses on Thursday as hot gases melted the glacier over the volcano. Sections of the country's main ring road were wiped out by the flash floods.
More floods from melting waters are expected as long as the volcano keeps erupting -- and in 1821, the same volcano managed to erupt for more than a year.
Small amounts of ash settled in northern Scotland and Norway, but officials said it posed little threat to health.
The ash cloud, drifting between 20,000 to 30,000 feet (6,000 to 9,000 meters) high and invisible from the ground, initially blocked the main air flight path between the U.S. east coast and Europe. On Friday, the cloud's trajectory was taking it over northern France and Austria and into eastern and central Russia at about 25 mph (40 kph).
Fearing that microscopic particles of highly abrasive ash could endanger passengers by causing aircraft engines to fail, authorities shut down air space over Britain, Ireland, France, Denmark, Norway, Sweden, Finland and Belgium. That halted flights at Europe's two busiest airports -- Heathrow in London and Charles de Gaulle airport in Paris -- as well as dozens of other airports, 25 in France alone.
Air space restrictions were lifted, imposed or extended Friday as the cloud moved.