Police arrested a man over a suspected arson attack on Friday after fire broke out inside a train at one of Hong Kong's busiest subway stations during rush hour, injuring at least 17 people, including two who are in a critical condition.

A 60-year-old man surnamed Cheung, who was seriously injured in the incident, admitted he started the fire while the train was headed to the busy Tsim Sha Tsui shopping district, district Police Commander Kwok Pak-chung told reporters.

"While he was on the way to the hospital he mentioned some personal reasons, but he was incoherent. What he said did not make much sense," Kwok said.

He said there was so far no information to suggest the incident could be described as an act of terrorism. The Counter Terrorism Response Unit which was patrolling nearby at the time of the incident was at the scene quickly to assist, he said.