Fire officials extricated the corpses of five men and a woman from under the rubble of the building located near Taljai hillock, they said.
Rescue work was still on at the site to trace five more persons suspected to be trapped under the debris, fire officials said.
According to a Pune Municipal Corporation (PMC) spokesman, the building was an unauthorised construction and a notice had already been served on its owners for demolition.
Municipal Commissioner Mahesh Pathak said PMC would file a complaint against the owners following the mishap.
According to sources, most of the victims were construction workers staying in the precarious structure.
Chief Fire Officer Prashant Ranpise said that the accident took place when the slab was being placed on the under-construction structure. "We got a call around 1.45 noon. The building was under construction. The slab work was going on...As labourers were working, the building collapsed," he said.
Photos: Tamannaah Bhatia sizzles in metallic gown
Photos: Arjun Kapoor and Raj Thackeray at event in Mumbai
In pictures: Here are the fastest ODI centuries in cricket history
'Shaadi No. 1': 10 famous Bollywood weddings in pictures
Photos: Shah Rukh Khan, Shweta Bachchan at Karan Johar's book launch