Airspace fully closed in Britain, Belgium, Denmark, Finland, Ireland, the Netherlands, Norway, Sweden and Switzerland.
Partial closures in Germany, Poland and France, which has closed 24 airports.
Brussels-based Eurocontrol, the agency that coordinates flights in Europe, says half of the average 600 daily trans-Atlantic flights may be cancelled by Friday.
International flights, including from Tokyo, Hong Kong, Dubai and Athens, have been affected by cancellations in northern Europe.
Emergency officials in Iceland were evacuating hundreds of people late on Thursday as new flash floods threatened to endanger farms in the area near an ash-spewing volcano.
Heathrow airport, Europe's busiest, was among the British airports that were to be shut down from 1100GMT, authorities said.
In London, Britain's air traffic service said it was extending a ban on most air traffic in England until 1200GMT Friday, but flights to Scotland and Northern Ireland may be allowed to resume.
Half of all transatlantic flights were expected to be cancelled on Friday due to the cloud of ash, the intergovernmental Eurocontrol agency said.
On average, there are around 600 flights daily between Europe and North America.
Al Jazeera's Nadim Baba, reporting from London Heathrow, said the disruption had caused chaos at the airport which normally sees 1,200 flights in and out a day.
He added the decision to cancel flights was due to the ash affecting flight visibility and also the workings of plane engines.
|Hundreds of flights have been cancelled in Britain, Norway and Denmark [Reuters]
"There have been incidents in the past where planes have had to shut down all four engines if they've gone through clouds of volcanic ash. So it's an extreme worry safety-wise," he said.
Leslie Tangen, a reporter for Norway's TV2 channel at Oslo airport, told Al Jazeera passengers had been left distraught at the cancellations.
"Some people have left the airport crying. They don't know what to do because they can't reach their destinations," he said.
"Tens of thousands of people are going to have problems because of this today."
Steff Gaulter, Al Jazeera's senior meteorologist, said the ash could threaten airspace until at least Monday.
"To clear the ash we either need the volcano to stop belching debris or we need a change in the weather.
Days of disruption
"Unfortunately, we have a high pressure just to the south of Iceland. This one is a 'blocking high', a high pressure that has essentially got stuck, and forces all the other weather to go around it.
"This means that if the volcano continues to throw ash up into the atmosphere, then ash still threatens to still be a problem, off and on, at least until Monday."
|The volcano has sent a plume of ash several kilometres into the air [AFP]
However Icelandic airports have remained open, with winds blowing ash away
from the island.
"Flights to and from Iceland are still OK. The wind is blowing the ash to the east," Hjordis Gudmundsdottir of the Icelandic Airport Authority told AFP.
"It's amazing really. Things here should be fine for the next 12 hours at least, and we think probably all day, judging from the weather forecast."
On Wednesday between 700 and 800 people were evacuated from their homes in the remote, lightly populated area 125 km east of the capital, Reykjavik, as the volcano melted a glacier, causing massive flooding.
An Icelandic geophysicist said that the volcanic eruption could last "a long time".
"It is very variable how long these eruptions last. Anywhere from a few days to over a year," Magnus Tumi Gudmundsson, a professor of geophysics and civil protection adviser, told the AFP news agency.
Last month, the first volcano eruption at the Eyjafjallajokull glacier since 1823 - and Iceland's first since 2004 - briefly forced 600 people from their homes.