Shoppers and restaurant customers were among those still inside the World Trade Centre, a retail and office complex in the city's Causeway Bay district, Hong Kong police said.
The fire started in the early afternoon at the building on Gloucester Road, the government said. The 38-storey complex houses offices, restaurants and a shopping centre.
Twelve people felt unwell and were taken to hospital, the authorities said.
Local broadcaster RTHK earlier reported that police said about 100 people were moved from a restaurant to a podium on the top floor when the fire broke out and smoke filled the dining area.
About 300 people in total were moved to the top of the building while waiting to be rescued, police told local media.
Firefighters used an extendable ladder to rescue the people on the fifth floor, where tens of people were jammed into narrow areas on an open-air podium. Other people were said to be trapped in restaurants in the mall, added a local newspaper.
About 1,200 people were rescued from the building, according to local media reports.
Authorities said that the fire began on the first and second floors of the building, and that the first to fifth floors of the mall had been undergoing extensive renovation.
The fire services system had also been shut down, according to the Hong Kong Fire Services Department’s senior divisional officer Ng Yau Sheung.
“If the system is under repair, it’s possible that the contractor will shut down the area affected,” he said during a news conference Wednesday evening.
The fire was given a level-three incident rating on a scale of five, the government said.
Preliminary investigations by the police suggested that the fire started in an electrical switch room, located on the first floor of the shopping mall, before spreading to scaffolding that surrounded part of the building, the South China Morning Post reported.
The building is owned by Sun Hung Kai Properties Ltd.
Reuters contributed to the report.