What should citizens do when a building collapses?

On Friday, Hong Kong saw a deadly collapse of a five-storey building of 50 years in To Kwa Wan, a residential district living mainly grass roots. The sudden downfall shocked the neighborhood, as well as the rest of the city, at around 1pm, and so far three people were confirmed to be killed and two others injured.

According to witnesses, a renovation was taking place on the ground floor shop during the accident. And experts believe external damages are more likely to have caused the disaster than the constructional problems of the aged building.

Residents of the building, which is now nothing more than a heap of ruins, and buildings around the area are provided with temporary shelters at community centers in the district. 

Unlike Mainland China, such a fatal disaster is still an unbelievable fact to the city. How on earth should people do?