Citybus said one of its bus drivers who was struck in the chest by a metal rod while behind the wheel has been discharged from hospital.

The incident happened on Monday evening, when a metal rod pierced the windshield of a double-decker bus travelling on Tsing Long Highway.

An investigation is under way into how it happened.

Citybus released an audio message from the driver, who expressed gratitude to members of the public for their support.

"I have been discharged from hospital. Thank you for your concern. I am fine now," he said.

"I just can't move too fast for the time being.

"I am especially grateful to my wife for her concern and my colleagues for their continued regards. I will continue to serve the public and hope to return to work as soon as possible."

Citybus said it would offer appropriate assistance to the driver and continue to follow up on the incident with relevant authorities.

Police earlier arrested a truck driver on suspicion of dangerous driving.