The busiest hub of the nation's busiest airport, O'Hare International, slowed to a harried crawl today as the United Airlines pilots' strike took hold in the airline's home base.

Three dozen of the 313 scheduled daily United departures made it into the air today, causing delays for thousands of passengers. Terminal Two, from which almost all of United's flights depart, was a scene of mild confusion and complaint, and sometimes great frustration.

Concourses E and F, normally bustling with thousands of United passengers any hour of the day, were almost empty. United's flights usually account for 50,000 seats a day out of O'Hare -- about 37 percent of the total handled by the airport. But today, most of those travelers scrambled to find seats on competing airlines or to map out zigzag itineraries. Dozens of United supervisors offered assistance to luggage luggers and pram-pushers and the others who gathered in perplexity to study the arrival and departure screens over the United ticket counters.

Outside the terminal, hundred-thousand-dollar-a-year aircraft commanders in full uniform marched on the pavement, their "Management Unfair to Labor" signs flapping from their necks in a brisk spring breeze.

Inside, TV crews interviewed travelers and anchormen practiced their news-story introductions. And amid all these distractions, there were genuine concerns.

Swedish businessman Lennart Sivertsson, 37, fretted that he would miss a connection to Europe because his 4 p.m. United flight to New York had been delayed at least an hour. And at the other end of his long journey home, he said with a rueful look, a Swedish control-tower strike was under way. "I'll have to fly to Copenhagen, and then see how I get home from there," he said.

Peter Stein, a Chicago physician, and Diane Camp, 26, a nurse, were trying to get to Philadelphia where they planned to talk over wedding ideas with her parents. But their United flight, scheduled to depart at 8 p.m., had been canceled at 9 a.m. this morning and they were searching for seats on another flight.

"We had enough to do without this happening," they said as they waited to learn whether they could get aboard any rival airline's flight.

A Chicago couple was headed to New York for a surprise party marking a relative's 25th wedding anniversary. Somehow, they had missed the news that United had been struck last night after management and the pilots could not agree on a new contract.

"Just as we got here, we saw the pilots," said the woman, who asked not to be identified. "We didn't know and now we're very disappointed and trying to find out whether we can make the surprise party at all."