Hong Kong  - An investigation was under way Friday into how concrete and tiles showered down from a Hong Kong high-rise office block, injuring four people and damaging two vehicles.

Lunchtime crowds were hit by shards of cement and tiling when a three-metre long slab fell off the eighth floor exterior of the building in the city's busy Wan Chai district.

The concrete hit a canopy on the way down, breaking into smaller pieces before crashing on the road beneath the 25-year-old office block on Thursday afternoon.

Three men and a woman were injured by the falling rubble. Three of them were taken to hospital with minor injuries while a third was treated at the scene. Two vehicles were also damaged.

Investigators from the city's Building Department were Friday looking into the cause of the incident which is believed to have been triggered by torrential downpours in recent days.

Hong Kong has been deluged with the heaviest June rains in living memory, causing extensive flooding and landslides across the city of 6.9 million over the past fortnight.

Building owners in the high-rise city face heavy fines for failing to maintain the exteriors of office and apartment blocks. Hong Kong also has specific criminal offence of throwing objects from a height. (dpa)