DALLAS – Southwest Airlines says hundreds of flights have been delayed by technical issues that are forcing it to check in some customers manually at airports and are causing long lines.

The Dallas-based company is asking travelers to arrive at least two hours before their scheduled departures as the problems that began Sunday morning continue into the evening. It is also asking customers to use airport kiosks to print boarding passes and tags for luggage.

At Los Angeles International Airport earlier in the day, several dozen people crowded the Southwest terminal waiting to be issued hand-written tickets. By late afternoon, Southwest said about 450 of the 3,600 flights scheduled for the day had been delayed.

E.J. Schultz, a reporter for Ad Age who was taking a Southwest flight from Chicago’s Midway Airport, said the airline was telling people at the gate that travelers with paper boarding passes were fine. But those who had downloaded their passes onto their mobile phones were told they had to stand in line, he said.

Schultz said he didn’t understand why Southwest didn’t announce that people should print out their boarding passes at home before getting to the airport.

“If everyone had done that, it would’ve saved so much time,” he said.

Schultz said there was a line of about 50 people at the Southwest gate at around 4 p.m. local time. His flight had not yet been delayed.

The long lines at check-in may mean some passengers didn’t make their flights.

Emily Mitnick, who was flying to Detroit from Denver International Airport, said she missed her 10 a.m. flight, even though she parked her car around 8 a.m. She estimated that about 1,000 people were on line at the check-in for a boarding pass. When she went downstairs to the curb-side check-in, she said there were about a couple hundred people in line there as well.

By the time she got on line to go through security, it was around 10:15 a.m.

“The clock was ticking and the flight took off,” said Mitnick, who was trying to get to Detroit through a different flight to Chicago.

In a statement, Southwest said it was still having “intermittent” technical issues on its website, mobile app and in its phone centers and airports check-in systems. It said that while it is working on the issues, workers at airports are helping customers with their itineraries.

Representatives for Southwest did not say what caused the problem, or how much longer it was expected to last.

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