Passengers stand in line for security checks at Terminal 3, Beijing Capital International Airport, on Sunday afternoon. [Photo/ China Daily]

China's civil aviation authority yesterday raised security levels at airports.

Passengers are now asked to remove shoes and belts during security checks and more luggage is opened.

The Civil Aviation Administration of China didn't give any reason for the increased security and passengers hadn't been notified in advance but there was speculation the move was related to the recent discovery of knives and other banned items at an airport in the Xinjiang Uygur Autonomous Regions in the country's northwest.

Similar security measures were put in place during the 2008 Olympic Games in Beijing and the World Expo in Shanghai last year.

The sudden imposition of stricter checks caused problems at Beijing Capital International Airport with long queues, arguments and even scuffles breaking out among passengers having to wait a much longer time to get through the security checks.

"The longest journey in the world is from the security checkpoint to the boarding gate and the most painful thing in the world is that you finish the journey but the plane door has closed," one passenger complained on his microblog. He missed his flight and had to wait for the next one.

Photos posted online showed the airport crowded with passengers in scenes that were described as like a "busy and noisy railway station during the annual spring rush."

Passengers said it took them more than an hour to get through security and about half of them had their luggage opened and subjected to detailed inspections.

There were no problems in Shanghai, however, as more security channels were opened and the checks went smoothly, airport officials said.

They said the level of checks was raised according to the requirements of the civil aviation authority but "no passengers missed flights because of the checks."

Security staff will not demand that all luggage be checked in order to maintain passenger flow, they said.

Normally passengers need to wait no more than 10 minutes before passing through the checks at Pudong International Airport and no more than 15 minutes at Hongqiao International Airport, the officials said.

However, airline companies are advising passengers to arrive at airports two to three hours ahead of their departure time rather than the previous one to two hours.

On August 3, security staff at Xinjiang's Urumqi airport found a knife hidden in a water bottle and another one in an umbrella a few days later.

Around the same period, other banned items such as inflammable goods and electric shock devices were also seized from passengers.