"11 floors seem to have been completed so far. The building collapsed suddenly according to statement of eyewitnesses," Fire and Rescue Services Joint Director S Vijayasekar told PTI.
One worker has been rescued, he said adding ambulances, doctors and paramedical personnel were on the spot.
Twelve fire tenders and 100 personnel were involved in the rescue work, he said.
The incident occurred at Moulivakkam near Porur in the western suburb, about 20 km from here, as rains lashed the area and several other parts of the city this evening.
On the number of trapped workers, he said "it is not possible to categorically comment on it..Eyewitnesses say that there were about 50 construction workers at the time of the collapse.