Passengers stand in line for security checks at Terminal 3, Beijing Capital International Airport, on Sunday afternoon.

Fights broke out among angry passengers at Beijing Capital International Airport on Sunday morning, after the security level for domestic flights was raised from level one to level two.

Long lines formed as passengers were told to remove their shoes and belts and luggage was checked, and tempers flared as passengers pushed and shoved to get through the security gates.

"I have witnessed three fights," said one passenger on his micro blog.

The higher security level at Terminal 3 caught thousands of passengers and the airport unprepared, with many flights delayed.

Beijing Capital International Airport has refused to comment on the reason for the extra security checks.

The crowd finally began to disperse around 10 am, according to Chen Ming, a staff member at the information desk.

Chen said that the raised security level will continue for at least the next few days.

One member of staff doing the safety checks said on condition of anonymity that the intensified security checks were only directed at flights to certain directions, but refused to provide further details.

"Standing in long lines is not truly dreadful. What is dreadful is that there is no explanation," wrote Liu Wei, vice-president of Legend Capital on his Sina micro blog.

Meanwhile, passengers are being reminded to allow extra time for the security checks, especially for morning flights as the longer waiting times are expected to continue.