LONDON (Reuters) - Dozens of flights at Heathrow Airport's new Terminal 5 were cancelled on its opening day on Thursday after problems with security and luggage.

At least 33 short-haul flights were cancelled, British Airways (LSE: BAY.L - news) said.

Passengers, whose flights did go ahead, were left stranded for hours as crews struggled with the baggage handling system.

Reuters journalist Mark Jones was left sitting on a plane for two hours.

"At one point the luggage was returned to the terminal because the system said our flight had already departed," he said.

British Airways, which now has its home at T5, said other problems at the 4.3 billion pound terminal included car parking and staff security screening.

"This is not unexpected following one of the most complex and largest airport moves in history," it said in a statement.

"These teething problems have included car parking provision, delays in staff security screening and staff familiarisation with the terminal. We have also had some baggage performance issues.

"These issues are being resolved and we are sorry for any inconvenience this has caused."

Airport operator BAA was not immediately available for comment.

Tight security had been put in place in expectation of demonstrations by green protesters against the expansion of the world's busiest international air gateway.

About 200 demonstrators appeared in the main entrance wearing Stop Airport Expansion T-shirts.

Authorities had been on alert after a series of security breaches at Heathrow.

On the eve of the Queen formally opening the terminal this month, a man ran onto a runway, while in February, protesters broke into the airport and wrapped a banner around a plane.

The terminal is the equivalent size of about 50 soccer pitches.

The first passengers to arrive at the new terminal were from Hong Kong at 4:50 a.m. local time, while the first outbound plane left the airport at 6:20 a.m. bound for Paris.

(additional reporting by Andrew Hough; Editing by Steve Addison)