"The extent is greater than we've ever seen before in the EU," said Brian Flynn, deputy head of operations at Eurocontrol, the European air traffic control organisation.
"The meteorological situation is such that the volcanic ash is progressing very slowly eastwards but there is not a lot of wind... so it is very slow and very dense."
'Wait and see'
UK airspace was shut down to all but emergency flights from midday (1100 GMT) on Thursday, and will not reopen until 0700 BST (0600 GMT) on Friday at the earliest. It was also closed in the Republic of Ireland.
Republic of Ireland
Partial or planned closures:
Sweden (total closure by 2000 GMT)
Denmark (total by 1600 GMT)
Finland (northern airspace closed till 1200 GMT Friday)
Belgium (total from 1430 GMT)
Netherlands (being shut progressively)
France (northern airports by 2100 GMT)
"Volcanic ash represents a significant safety threat to aircraft," said the UK's Air Traffic Control Service (Nats).
Oslo airport, which is Norway's largest, was closed on Thursday morning, meaning Norwegian airspace was completely closed.
"Flights will be cancelled probably all day with the current prognosis," said Jo Kobro, information manager at Oslo Airport. "Then we have to wait and see what the new weather forecasts will say about the wind direction, and if we are lucky the volcano diminishes in strength."
Tim Farish, who had been planning to fly from Oslo to London on business, said he had been told by the airline SAS to stay at home and not bother calling for updates.
"We can actually smell sulphur in the air here now from the volcano cloud," he told the BBC from his home in the Norwegian capital. "This could last for a few days apparently, so all I can do, like anyone else, is sit and wait."
Stranded passengers in Paris, Stockholm, Dublin and Oslo
Belgium, Sweden and Denmark announced they would be shutting their entire airspace, while northern Finland's airspace was closed and the Netherlands' was being closed progressively.
French aviation officials said on Thursday afternoon that the main airports in Paris and other airports in the north of the country were to be closed.
There was severe disruption in Germany and in Spain.
Road cut off
The last volcanic eruption beneath the Eyjafjallajoekull glacier in south-western Iceland was on 20 March, and it forced about 500 people in the sparsely populated area from their homes. Before that, the previous eruption started in 1821 - and lasted for two years.
A TV crew drove through the Icelandic ash cloud during daylight hours
Eyewitnesses and local officials said two flows of floodwater had started coming off the glacier on Wednesday, and that a road along the flooded Markarfljot river had been cut in several places.
As many as 800 people were evacuated from their homes.
On Thursday, the flooding was reported to have subsided, but the volcano was still producing ash that was being blown towards Europe.
"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," Einar Kjartansson, a geophysicist at the Icelandic Meteorological Office, told the Associated Press.
"It depends how the wind carries the ash."
Iceland lies on the Mid-Atlantic Ridge, the highly volatile boundary between the Eurasian and North American continental plates.
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