Hundreds of flights worldwide were cancelled or delayed as volcanic ash from Iceland swept across northern Europe, closing airspace above Britain, Ireland, Scandinavia and other countries.
Thousands of passengers from Hong Kong to Dublin were stranded as aviation chiefs decided it was too risky to allow planes to fly through the cloud of ash, which is upwards of 6km above the earth's surface.
The ash from the volcano under Iceland's Eyjafjallajokull glacier blew southeast after Wednesday's eruption towards Scotland and Norway, before covering Britain and Scandinavia, according to the London-based Volcanic Ash Advisory Centre.
Although not visible from the ground, volcanic ash can be highly dangerous for aircraft, clogging up the engines and reducing visibility, experts warn.
Norway was the first to ground its flights on Wednesday evening, followed by Scotland overnight and then London, before air traffic controllers announced no flights could go through British and Irish airspace after 1100 GMT (2100 AEST).
Finland closed its northern airspace an hour later, although Helsinki-Vantaa airport remained open, and Denmark and Sweden said they would follow suit.
Belgian air space had been partially closed and would be shut down entirely from 1430 GMT (0030 AEST, Friday) an official said, and Dutch air traffic control said the Netherlands air space was being progressively closed.
Earlier, about 40 to 50 flights were cancelled at Amsterdam's Schiphol airport, all bound for Scandinavia, Britain and parts of Russia, while delays were also reported on US and Canada-bound flights.
Icelandic airports, however, reported no problems.
"The wind is blowing the ash to the east," Hjordis Gudmundsdottir of the Icelandic Airport Authority said, adding: "It's amazing really."
About 300 flights in and out of London's Heathrow and Gatwick airports had already been cancelled before the airspace was closed, leaving many of the 260,000 passengers that typically use the airports each day with nowhere to go.
Flights from all over the world, including Tokyo, Hong Kong, Dubai, Paris and Athens, were affected by the cancellations in northern Europe.
A spokeswoman for Britain's air traffic control service said the airspace closures could also cause European flights to the United States to be re-routed.
"A lot of traffic from Western Europe to America would normally fly through our space," she said.
Some airlines raced to get their flights completed before the cloud hit - Pakistani International Airlines diverted a flight from Lahore to Manchester in northwestern England to London, just before the capital's airports shut.
"It will be difficult to fly to UK and other European countries affected by the ash cloud until it settles completely," said PIA spokesman Sultan Hasan.
British airspace was only supposed to be closed until 1700 GMT (0300 AEST, Friday), although the airline BA said it was grounding all its British flights until Friday morning.
David Rothery, a senior lecturer in earth sciences at Britain's Open University, said flight restrictions were an essential safety precaution.
"This is because if volcanic ash particles are ingested into a jet engine, they accumulate and clog the engines with molten glass," he said.
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