According to software giant Microsoft, more than eight million computers have been affected by the "largest IT outage in history".
The company said in a blog post on Saturday that a software update from cybersecurity firm CrowdStrike had affected nearly 8.5 million Microsoft devices.
Worldwide, flights were halted, patients were unable to make appointments, train passengers were delayed and football clubs were unable to sell tickets. In addition, broadcasts from media companies were severely disrupted.
Microsoft said in a Reuters report: "We currently estimate that the CrowdStrike update affected 8.5 million Windows devices, or less than 1 percent of all Windows machines."
One expert called it the "largest IT outage in history."
The 2017 WannaCry cyberattack is known as the closest attack to CrowdStrike, with an estimated 300,000 computers in 150 countries affected.
That same year, the NotPetya attack affected more than 2,300 organizations in over 100 countries, causing an estimated loss of $10-11 billion.
Ciaran Martin, former head of the National Cyber Security Centre, part of Britain's GCHQ intelligence agency, said the scale of the problem was enormous.
"This is not unprecedented, but I can't imagine a failure on this scale," he said. "It has happened over the years, but this is one of the largest. I think it will probably be short-lived, because the nature of the problem is actually quite simple."
Hundreds of displaced airport passengers have flocked to the Port of Dover in an attempt to circumvent the chaos caused by the global computer outage.
Authorities have warned people not to arrive at the port on Saturday without a reservation, as thousands of families struggle to reach Europe at the start of the summer holidays.
About 200,000 people will face delays and cancellations as a result of Friday's IT meltdown, which was caused by a faulty update to a widely used piece of cybersecurity software. Many travel insurers are refusing to pay for rescheduling holidays.
NHS services, train services, cash machines and contactless payments were also taken offline, with 999 services experiencing major delays as GP practices were unable to book appointments.
Airports have said their systems are back up and running after global IT outages caused problems, but trips can still be affected by cancellations and delays.
This has led to many travelling to the Channel Port in the hope of making the crossing to the mainland.
However, the Port of Dover has warned that passengers were already facing a 60-minute wait to get through border controls on Saturday morning, despite traffic being handled well.
The global IT outage was caused by a bug in the cybersecurity software of the American company CrowdStrike. As a result, millions of computers running Microsoft Windows suddenly crashed on Friday morning.
It resulted in a so-called 'blue screen of death', which put the computers into recovery mode.
On Saturday morning, P&O Ferries warned of heavy traffic that would lead to congestion on the roads to the port.
They advised passengers to allow extra time during the journey and to bring refreshments.
Irish Ferries also advised passengers to allow sufficient time for border controls and check-in.
DFDS warned on X, formerly Twitter, of waiting times of up to 120 minutes at border controls and 30 minutes at check-in. Dover advised passengers to ensure they had sufficient provisions with them, such as water, food and prescribed medication.
At Dover, Operation Brock, a system to manage the traffic of lorries queuing to cross the Channel, was installed on the M20 towards the port.
Travel organisation Abta urged holidaymakers to check with providers whether there are any additional measures they need to take.
A spokesperson said: "We are at the start of one of the busiest travel periods as some schools finished their summer holidays yesterday and many more continue next week.
"Many people will be flying abroad to escape the unusual weather in the UK recently.
"Spain, Turkey and Greece are among the popular destinations for an overseas trip.
"If you are going on holiday this weekend - in any form - it is advisable to check with your travel company whether there are any additional measures you need to take, as some businesses are still feeling the impact of Friday's IT outage."
Eurotunnel warned of delays of around 30 minutes to booked times at its Folkestone terminal on Saturday morning.
Airport passengers continue to face long delays when boarding flights to and from the UK.
Andrew Evans, from Taunton in Somerset, said he had been waiting at Palma de Mallorca airport for more than nine hours and claimed there had been little communication from airline TUI.
His flight to Exeter, which was due to depart at 2.05am local time (1.05am UK time), still had no passengers on board by around 10.30am on Saturday morning.
"Our holiday was completely ruined at the end. It felt like we were abandoned," said Evans.
Meanwhile, passengers on a Ryanair flight from Bristol to Portugal reported arriving at their destination but that no checked baggage had been loaded onto the plane.
TUI said its IT systems remained "unstable" and apologised to passengers.
There were also long queues to board at Heathrow. The British Airways app and the automatic check-in desk did not allow passengers to check in.
Chris Shaw, 61, a consultant from London, boarded a replacement British Airways flight from Heathrow to Berlin at 8.45am on Saturday after his original afternoon flight to the German city was cancelled on Friday.
"The row [for check-in] "If it had taken that long, we would have missed the flight, which was clearly overbooked," he said.
"So I pushed my way in and insisted that something be done to me. The flight was completely full, so if I hadn't pushed my way in, we wouldn't have even gotten seats.
"We arrived at the gate with 20 minutes to spare. Security was excellent and fast, but my criticism of Heathrow was the lack of information and the staff were very uninformed.
"There was no priority for urgent flight needs and there were not even notice boards telling passengers where to go or what to do."
Indie rock band Bombay Bicycle Club had to postpone a performance at the Poolbar Festival in the Austrian town of Feldkirch on Friday after their flight was cancelled due to the outage.
Transport Secretary Louise Haigh said airport IT systems were "back up and running and working normally".
"We are in constant contact with the industry," she said. "There are still no known safety or security issues arising from the outage. Some delays and a small number of cancelled flights are expected today."
'More preparation needed'
Experts say the chaos that followed the outage is a warning that societies should prepare for more large-scale IT chaos in the future.
Professor Ciaran Martin, former director of the National Cyber Security Centre, warned that countries "will have to learn to deal" with future failures.
He said: "The worst of this is over because the nature of the crisis was such that it went very wrong, very quickly. It was picked up quite quickly and in fact it was shut down."
Prof Martin told Sky News: "Until governments and industry work together and work out how we can address some of these flaws, I fear we're going to see more of these issues.
"Within countries like the UK and elsewhere in Europe, you can try to build that national resilience to deal with this. But ultimately a lot of this will be determined in the US.
"If there has to be regulation to fix these shortcomings, it will probably have to come from the US and there's not much we can do about that.
"So until the structure of how we use technology changes, we're going to have to learn to deal with these things, rather than eliminate them."
A computer scientist said people should learn the same lessons from the global IT outage as they did from the pandemic.
Sir Nigel Shadbolt told the BBC's Today programme: "Often these issues are overlooked [to] technological elite.
"This has implications for everyone and we need to understand how those effects ripple through society and think about how we can all make ourselves more resilient."
He added: "The resilience in general of these systems is something quite special. We depend on these systems and in general they operate at a very high level of quality.
"But if things do go wrong, and it looks like a pandemic, then we literally have to learn similar lessons. What lessons do we learn?
"What should we be thinking about as individuals? We should be thinking about a level of resilience in our own lives. We should be thinking about maybe having multiple systems, not just being dependent on one."
