Three people including a three-year-old girl were rushed to hospital on Friday evening after a fire at a tenement in Mong Kok that police believe was started deliberately.
A 33-year-old woman was held on suspicion of arson after the blaze, which broke out at about 6pm and forced the evacuation of about 20 people from the block on Tung Choi Street.
Police arrested the suspect on the corner of Sai Yeung Choi Street and Shantung Street, and she was held for investigation.
Firefighters quickly put out the blaze and rescued a dog from the property.
Three people including the child were taken to Kwong Wah Hospital for treatment for smoke inhalation.