Manchester firefighters were dispatched to Walnut St for a reported building fire on Walnut Street and could see a column of smoke leaving the station.   Companies arrived to find heavy fire showing from the rear of a large 2 1/2 woodframe multiple dwelling.  Command struck a 2nd alarm bringing in mutual aid to the city.  Companies made an interior attack but were drawn back to the heavy fire conditions.  Crew went defensive for a short period of time to knock down the heavy fire and the fire building's roof partially caved in.  Companies remained on scene for several hours overhauling and checking for extension.