LONDON — One of the world's busiest airports was thrown into chaos Thursday when a terrorism alert virtually shut down Heathrow, creating human gridlock with bewildered vacationers pushing mountains of luggage.
Officials at the airport said the call came about 2 a.m. that police had made arrests in connection with a suspected plan to blow up several U.S.-bound aircraft in midair.
By midmorning, with most flights halted, all four terminals of the huge airport were teeming with passengers trying to find a check-in line.
Harassed officials with clipboards and printed notices moved among the mass of passengers, trying to help and to explain draconian new security measures.
The measures dictated that all passengers abandon carry-on luggage and take nothing onboard except for essential travel documents, money and eyeglasses without their cases. Everything had to be carried in clear plastic bags, which staff members hurriedly distributed.
Terminal 3, from which most transatlantic flights depart, was a tangle of interlocking lines of passengers.
Charlotte Small was busy answering distraught passengers' questions, such as: "Where's the end of this line?"
Small usually works in customer services at the airport's duty-free shops, but she said they were all closed.
"We've been asked to help out," she said.
Many travelers had heard about the situation as they were on their way to the airport.
Heather Cobb, arriving at the airport after taking a cruise around Britain, heard the news aboard the bus.
"We've been here since 6 a.m.," she said at 10.30, still in line to check in for her flight to San Diego. She and her husband were resigned rather than angry.
"I understand why they're doing this," Cobb said of the security measures.
Most seemed to accept the delays with stoicism.
Duweihi al Duweihi was on his way to Riyadh, capital of Saudi Arabia, after a vacation in Canada with his wife and four children, one a baby clutching a bottle of milk.
He too was stoic. "We've been without sleep and traveling for around 24 hours," he said after a two-hour wait in the departure line at the security checkpoint. "I know they're going to test the baby's milk, but it's better to be safe."
Nami Bryan and her charges -- Japanese children returning from summer school in the cathedral city of Canterbury -- had been on their feet for hours.
"I've got 100 children to get back to Tokyo," she said. "We heard there was a problem, so we've been up since 5 a.m. and I don't know if we'll make our flight.... But the children are really good, not upset or frightened at all."
In Terminal 2, short-haul European travelers were told of delays, but eventually those in the check-in lines did an about-face and headed to ticket and information desks as airlines canceled their flights for the day. The airport was all but closed by midday.
By evening, airport staff said flights were beginning to take off.
The security alert remained high.
"Departures are operating -- with delays," said Matilda Gosling of Heathrow's media office. She said about two-thirds of the airport's 4,500 security personnel had been brought in to monitor the day's operations.
At a news conference outside the airport, Heathrow chief executive Tony Douglas said, "It was a very difficult set of circumstances. I would like formally to apologize for inconvenience to the passengers.
"This has been a very tough and difficult time for us. I believe we've responded professionally and quickly and we've tried to communicate openly of the nature of the changes."
The domino effect was widespread as airlines canceled or postponed flights to Britain, disrupting schedules at airports worldwide.
Thursday night, after airport staff had advised most people to give up and go home, trapped travelers -- the ones who had nowhere to go -- camped out in Heathrow's great indoors.