London’s Heathrow Airport is one of the busiest travel crossroads in the world. It handles about 1,300 flights a day, from more than 80 airlines. Nearly six million people and more than 130,000 tons of cargo moved through the London airport last month.

So what happens when a hub this size grinds to a halt?

First, the planes full of people and cargo already on their way to Heathrow have to find another place to land. Countless passengers have to find hotel rooms wherever they end up. Crews and pilots must take legally mandated rest. Airlines have to figure out how to reroute planes and cargo.

Ian Petchenik, the director of communications at Flightradar24, a flight tracking website, described it as “a million moving pieces that are all trying to get where they need to go.”

Most major international airlines operate flights into Heathrow, which Mr. Petchenik called the “crown jewel of most international airlines,” and it receives flights from the widest variety of air carriers of any airport in the world.

When Heathrow shut down early Friday, 120 flights were already heading there.

Some were able to land nearby, at other London airports or in Birmingham or Manchester. Others were redirected to continental European destinations like Amsterdam and Frankfurt. One British Airlines flight that left from New York’s Kennedy International Airport ended up in Reykjavik, Iceland. A Japan Airlines flight from Tokyo went to Helsinki, Finland.

In Taipei, the day’s only flight to Heathrow took off shortly after 9 a.m. The plane had only made it past Hanoi when it turned around and headed back to Taipei, according to data from Flightradar24.

But most flights ended up neither where they had started nor where they planned to go, setting off a global logistical scramble for hotel rooms and alternate flights as passengers tried to figure out how to make it to sightseeing tours, business meetings and family events.

Officials in London said they expected the disruption at Heathrow to continue for several days. It could also take days for airlines to rebook the hundreds of thousands of passengers they expect to be affected.

The shutdown of an important global transit hub for such a prolonged period of time is “unprecedented,” Mr. Petchenik said. A rare, similarly extensive disruption occurred in 2010 when a volcano eruption in Iceland grounded more than 100,000 international flights over several days.