Departing Southwest Airlines passengers wait in line at McCarran International Airport in Las Vegas on Sunday. Technical problems delayed Southwest flights nationwide.

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 causing long lines.

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

Representatives for Southwest did not say what caused the problem or how long it would take to resolve. Spokesman Brad Hawkins said there was “absolutely no indication now” that the problems were the result of hack.

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 International Airport, said the airline was telling people that travelers with paper boarding passes were fine. Those who had downloaded tickets onto their mobile phones were told to stand in line, he said.