DALLAS — Hundreds of Southwest Airlines flights across the country were delayed or cancelled Wednesday after technology problems prevented many travellers from checking in or boarding flights.

Southwest began having intermittent problems with several computer systems after an outage around 2 p.m. Central time.

Southwest briefly held planes at their gates, according to spokesman Brad Hawkins. Even after those planes resumed moving, delays piled up over the next several hours.

By late Wednesday night, the airline tweeted that it was still manually checking in passengers.

For about three hours, visitors to Southwest.com couldn’t buy tickets, check in for flights, or check their flight’s status. The site appeared to be working again by late afternoon, then crashed again.

Shortly before 10 p.m. Central time, Southwest said the outages had caused 600 to 700 cancelled and delayed flights. Tracking service FlightStats Inc. put the numbers much higher – 400 cancellations and 2,000 delays.

Southwest said that computer systems were gradually coming back, but it warned customers booked on Thursday flights to check their flight’s status online and arrive at the airport early because long lines were expected.

Anxious customers tweeted to Southwest that they could not check in for flights.

Leah Boyd and her husband, Matt, were flying to Providence, Rhode Island, but were held up at the Baltimore airport for three hours by mechanical issues with two different planes.

They finally boarded a plane, but after sitting at the gate for nearly an hour passengers were asked to exit because of the technology outage, Boyd said. Then the pilots reached the end of their shifts, so passengers waited for a replacement crew.

The Boyds ended up cancelling their reservations and planned to drive to Rhode Island on Thursday instead. Leah figured it would be hard to find seats on another flight.

“I’ve never seen so many people in the terminal,” she said. “All these people are going to be flying standby.”

Airlines have sprawling, overlapping and complicated technology systems, and even brief outages can cause thousands of passengers to be stranded for hours.

Last October, an outage caused about 800 Southwest flights to be delayed and forced employees to issue tickets and boarding passes by hand. The airline blamed a software application, and it recovered in about a day. United Airlines and American Airlines both had computer problems last summer but fixed the problems within a day.

Dallas-based Southwest Airlines Co. carries more passengers within the United States than any airline. However, it is far smaller than American, Delta and United when international traffic is included.