A huge cloud of volcanic ash from Iceland spread further across Europe on Friday, grounding thousands more flights in the continent's biggest air travel shutdown since World War II.
As the giant no-fly zone grew, Europe's air traffic control center predicted 17,000 flights would be canceled Friday.
And experts warned the fallout from the Eyjafjallajokull volcano in southeast Iceland could take several days to clear. Some countries now say they will not even consider allowing flights again until Sunday.
The volcano spewed out more smoke and ash, building up the cloud that is being blown towards Europe. The cloud now extends from the Atlantic to the Russian capital and from the Arctic Circle to Austria and Bulgaria in southern Europe.
Thousands of people were stranded in airports around the world as a global flight backlog built up.
Europe's three biggest airports — London Heathrow, Paris-Charles de Gaulle and Frankfurt — were closed by the ash, which is a threat to jet engines and pilot visibility.
Eurocontrol, the European air traffic control group, said only 11,000 of the daily 28,000 flights in the affected zone would take off Friday. At least half of the 600 daily flights between Europe and North America would be canceled.
About 6,000 flights to and within Europe were canceled Thursday.
Austria, Belgium, Britain, Czech Republic, Denmark, Estonia, Latvia, Lithuania, the Netherlands, Poland and Slovakia shut down all or most of their airspace.
Finland, France, Germany, Russia and Spain experienced major disruption, although Sweden and Ireland gradually reopened their airspace and Norway temporarily allowed some flights as the ash drifted away.
"Forecasts suggest that the cloud of volcanic ash is continuing to move east and southeast and that the impact will continue for at least the next 24 hours," Eurocontrol said in a statement.
Most aviation authorities promised a review on Friday, but Finland set the tone for the grim weekend, announcing no commercial flights before Sunday afternoon at the earliest.
Poland had considered delaying the funeral on Sunday of President Lech Kaczysnki because the cloud threatened the flights of U.S. President Barack Obama and other world leaders.
But a senior presidential aide insisted the ceremony would go ahead as planned in the southern city of Krakow, even though its airport was closed Friday.
Other engagements were hit however — Portuguese President Anibal Cavaco Silva was stranded in Prague after Czech airspace closed and the Spanish royal couple were prevented from attending celebrations of Danish Queen Margrethe's 70th birthday.
In Britain, airports including London Heathrow, the world's busiest international air hub, were deserted as operators warned travellers not even to turn up for booked flights.
British officials extended the ban on non-emergency flights in most of its airspace until 0000 GMT Saturday "at the earliest," although some flights in Northern Ireland and western Scotland will be allowed.
The Eurostar Channel tunnel rail service reported thousands of passengers rushing to get places on its London-Paris trains. It laid on three extra trains but still could not keep up with demand.
Baltic ferries also reported a surge in demand.
Debbie Eidsforth, 36, spent the night at Heathrow and was trying to get back to Adelaide in Australia via Hong Kong.
"I had paid 5,500 pounds (8,500 dollars) for my flights, but it doesn't matter what class you fly in, everyone's in the same situation," she said.
In Scotland, health officials warned that ash falling to the ground over northern Britain might cause symptoms such as itchy eyes or a sore throat.
Amsterdam's Schiphol airport prepared beds and meals for 2,000 stranded travellers and at Paris Charles de Gaulle, passengers slept on cafe benches. In Brussels, 200 Bangladeshis diverted from London were blocked in the airport for a second day as they had no visas to leave the complex.
The volcano on the Eyjafjallajokull glacier in southern Iceland erupted just after midnight on Wednesday.
The ash drifted at an altitude of about 5-6 miles (8-10 kilometers). Although it could not been seen from the ground, experts said it posed a major threat.
In the past 20 years, there have been 80 recorded encounters between aircraft and volcanic clouds, causing the near-loss of two Boeing 747s with almost 500 people on board and damage to 20 other planes, experts said.
Finnish fighter jets which flew through the volcanic dust on training flights Thursday suffered damaged engines, the air force said, warning that even short flights could be dangerous.