Updated at 1:57 p.m. Eastern.
An ash-spewing volcano in Iceland emptied the skies of aircraft across much of northern Europe on Thursday, grounding planes on a scale unseen since the 9/11 terror attacks. British air space shut down, silencing the trans-Atlantic hub of Heathrow and stranding tens of thousands of passengers around the world.
Aviation officials said it was not clear when it would be safe enough to fly again and said it was the first time in living memory that an ash cloud had brought one of the world's most congested airspaces to a standstill.
A scientist in Iceland said the erupting volcano could eject tons of ash into the air for days or even weeks, while meteorologists from the AccuWeather forecasting service in Pennsylvania said the current ash plume will threaten Europe through Sunday at the least.
"Depending what happens and what the cloud does, this could last a couple of days," Kyla Evans, a spokeswoman for air traffic agency Eurocontrol in Brussels, said of the air traffic suspensions.
Britain's air traffic service banned all but emergency flights until at least 0600 GMT (2 a.m. EDT) Friday. The abrasive, microscopic ash was drifting between 20,000 feet and 36,000 feet high over the Atlantic Ocean, close to the flight paths for most routes from the U.S. east coast to Europe.
Patrick Horwood of Britain's National Air Traffic Service said the closure of U.K. air space was unprecedented.
"People can't remember a time when it has been on this scale," he said. "Certainly never involving a volcano."
A volcano beneath Iceland's Eyjafjallajokull glacier began erupting Wednesday for the second time in less than a month, triggering flash floods and sending smoke and steam up to five miles into the air. Video showed spectacular images of hot gases melting the thick ice, sending cascades of water thundering down the steep slopes of the volcano. Rivers swelled 10 feet in hours.
As the ash cloud drifted south and east toward northern Europe - including Britain, about 1,200 miles away - authorities in Ireland, Denmark, Norway, Sweden and Belgium also closed their air spaces.
The volcanic ash poses a threat to aircraft because it can affect visibility and can get sucked into airplane engines, causing them to shut down.
Evans said the ash had forced the cancellation of about 4,000 of the 20,000 daily flights across Europe.
France shut down 24 airports, including the main hub of Charles de Gaulle in Paris. Several U.S. flights bound for Heathrow had to return to their departure cities or land elsewhere when London airports were closed, including flights from Chicago, San Francisco, Denver, Las Vegas and New York.
In Washington, the Federal Aviation Administration said it was working with airlines to try to reroute some flights around the huge ash cloud, which is hundreds of miles wide.
Flights from Asia, Africa, the Middle East and Latin America to Heathrow and other top European hubs were also put on hold.
At Heathrow, which normally handles more than 1,200 flights and 180,000 travelers a day, passengers stared forlornly at departures boards on which every flight was canceled.
"It's like a tomb up there. Thousands upon thousands upon thousand of people waiting in long lines trying rebook flights from an airline that can't tell them when the planes are going to be flying again," reports CBS Radio News correspondent Vicki Barker from Heathrow.
"We made it all the way to take off on the plane ... They even showed us the safety video," said Sarah Davis, 29, a physiotherapist from Portsmouth in southern England who was hoping to fly to Los Angeles. "I'm upset. I only get so much vacation."
Eurostar train services to France and Belgium and cross-Channel ferries were packed as travelers sought ways out of Britain. P&O ferries booked a passenger on its Dover-Calais route who was trying to get to Beijing - he hoped to fly from Paris instead of London. Hours later, however, all Paris airports were shut down.
Everyone from tourists and business travelers to politicians and royals was caught up in the mayhem. In Britain, the closures curtailed some campaigning for the May 6 national election. Monarchs from Norway and the Netherlands traveling to a 70th birthday celebration for Denmark's Queen Margrethe found their plans upended.
Such widespread disruption to flights has not been seen the Sept. 11, 2001 terror attacks.
"There hasn't been a bigger one," said William Voss, president of the U.S.-based Flight Safety Foundation, who praised aviation authorities for closing down airspaces.
The National Air Traffic Service said Britain had not halted all flights in its space in living memory, although many were grounded after Sept. 11. Heathrow was also closed by fog for two days in 1952.
Gideon Ewers, spokesman for the International Federation of Airline Pilots Associations, attributed the extent of the disruption to amount of air traffic in the area.
"Normally, these volcanic eruptions affect air travel in areas of thin traffic such as the Aleutian islands in Alaska, or in Indonesia and the Philippines," he said.
The U.S. Geological Survey said about 100 aircraft have run into volcanic ash from 1983 to 2000. In some cases engines shut down briefly after sucking in volcanic debris, but there have been no fatal incidents.
In 1989, a KLM Royal Dutch Airlines Boeing 747 flew into an ash cloud from Alaska's Redoubt volcano and lost all power, dropping from 25,000 feet to 12,000 feet before the crew could get the engines restarted. The plane landed safely.
In another incident in the 1980s, a British Airways 747 flew into a dust cloud and the grit sandblasted the windshield. The pilot had to stand and look out a side window to land safely.
In Iceland, most of the 800 people evacuated Wednesday when the volcano began erupting were allowed to return to their homes. Farmers were advised to keep livestock indoors to protect them from eating the abrasive ash.
"It is likely that the production of ash will continue at a comparable level for some days or weeks. But where it disrupts travel, that depends on the weather," said Einar Kjartansson, a geophysicist at the Icelandic Meteorological Office. "It depends how the wind carries the ash."
Iceland, a nation of 320,000 people, sits on a large volcanic hot spot in the Atlantic's mid-oceanic ridge, and has a history of devastating eruptions.
One of the worst was the 1783 eruption of the Laki volcano, which spewed a toxic cloud over Europe with devastating consequences. At least 9,000 people, a quarter of the population of Iceland, died, many from the famine caused by the eruption, and many more emigrated. The cloud may have killed more than 20,000 people in eastern England and an estimated 16,000 in France.
There was no sign of the ash at ground level in Britain on Thursday, and officials said it posed little risk to human health. Weather forecasters said the most visible effect would likely be a spectacular red sunset.