Airlines operating flights into and out of the UK were forced to make major changes to their schedules after police said they had foiled a large-scale terror plot to attack planes heading from the UK to the US.
Long queues have been reported at Dublin Airport's check-in desks
Stringent security measures in place in the UK caused knock-on delays in airports around the world as passengers whose flights were not cancelled underwent thorough searches of their bags and belongings.
Scores of short-haul flights into London's Heathrow airport were cancelled on Thursday morning, but UK airport operator BAA lifted a blanket ban on short-haul travel in mid-afternoon.
British Airways cancelled all short-haul flights out of Heathrow on Thursday, and many long-haul flights suffered delays.
On Friday, British Airways says it expects to operate about 60% of short-haul and domestic flights, and 75% of long-haul flights, out of Heathrow. More flights, though not all, are expected to get the go-ahead from Gatwick.
All short-haul flights are expected to operate from other airports, though there may be delays.
Major US airlines also suffered from the alert: American Airlines said it cancelled three of its 16 daily flights from Heathrow to the US, and three flights from the US to Heathrow, but the remaining 13 US-bound flights from Heathrow went ahead.
The US has raised its terror alert to red, the highest level, for commercial flights originating in the UK.
It has also raised the alert level to orange, the next-highest level, for all domestic and international flights, and banned passengers from taking liquids or gels aboard other than baby food.
In Europe, budget airline EasyJet cancelled all flights in and out of all London airports, citing increased congestion.
On Friday, it said flights to a limited number of British and European destinations, detailed on its website, would be cancelled, but that it hoped to operate an otherwise normal service.
The German carrier Lufthansa confirmed that Heathrow-bound flights had been stopped, with three flights sent back to Germany and about 25 others cancelled.
Later on Thursday it said flights were operating again, but with delays.
Air France cancelled almost all of its Heathrow-bound flights. Spanish airline Iberia cancelled most flights to Heathrow on Thursday, but said it would be operating a larger plane between Madrid and London on Friday to enable delayed and scheduled passengers to travel.
Greece's Olympic airline suspended all flights to the UK for part of the day, and no flights at all left Athens for Heathrow.
Italy's national carrier, Alitalia, said it was grounding all flights to the UK, while aviation authorities in the Netherlands cancelled all flights to the UK until 1800 GMT.
Air New Zealand warned of delays to flights to London over the coming days.
Australian airline Qantas restricted hand luggage on UK-bound flights and banned liquids on flights to the US.
Flights from Israel to London airports other than Heathrow experienced delays, a spokesman told AFP news agency.
Germany's Frankfurt International Airport said it could accept some flights diverted from Heathrow.