HONG KONG • The 3,500 sq m roof of an indoor sports hall at Hong Kong's City University collapsed yesterday, injuring three people, officials said, sparking panic among eyewitnesses, who said it sounded like an explosion had gone off.

The incident happened around 2.30pm at a university sports centre in the Kowloon Tong neighbourhood, with footage showing the caved-in roof along with police, firefighters and ambulances at the scene.

"The roof collapsed and three people were injured," a police spokesman told Agence France-Presse.

Two men and a woman who were hurt have been taken to hospital, police said. The men, aged 55 and 57, are security guards, while the woman is a canteen supervisor.

The condition of the younger man was said to be serious. The 43-year-old woman and 57-year- old man were reportedly in stable condition, said broadcaster RTHK.

The indoor sports hall is used for sports, such as basketball, badminton and volleyball, and as an exam hall.

A student at the university told RTHK that exams were held there just three or four days ago for a large number of students.

An eyewitness told local broadcaster Cable TV that some of those nearby initially thought that there was an explosion.

"Someone ran out of it and screamed, 'Explosion, explosion'," he said. "The exam season was over, otherwise it would have been full of students as it is used as an examination hall."

According to City Broadcasting Channel, a student-run website, there was a complaint about falling objects during an exam last week, but the test went ahead.

Student Venkat Saraenji, 30, was eating at a canteen nearby when the collapse happened.

"It was quite loud," he told South China Morning Post. "I heard the sound, like 20 seconds of sound. I didn't know what happened... then I left the building."

A student, who gave only his surname, Tsui, told RTHK he was inside a canteen nearby when the roof collapsed.

He said students there felt the ground shaking, and some shouted "something is exploding" before running away.

City University said it will set up a three-person committee to investigate the collapse.

Education sector lawmaker Ip Kin Yuen has called on the university to inspect all of its other buildings as well to ensure the safety of students and staff, reported the Post.

Such accidents are very rare in Hong Kong, a city of about seven million people.