Officials of the Brihanmumbai Municipal Corporation (BMC) had put the death toll at four and the number of injured at nine. Maharashtra Housing Minister Radhakrishna Vikhe Patil earlier said 12 to 13 people were killed, citing preliminary information.
As night fell on the city, hundreds of people continued the job of rescuing people from under the rubble of the Kesarbai building, which housed an eatery on the ground floor and was located in a bustling lane off Tandel Street.
The difficult access to the site prevented ambulances and earth movers from being deployed, and rescue workers, including residents, used bare hands to remove concrete chunks, door frames and household items. Ambulances had to be parked some 50 metres away.
Chief Minister Devendra Fadnavis said the building was around 100 years old. It was not in the list of dilapidated buildings and was given to a developer for redevelopment.
Between 10 and 15 families lived in the building, he said.
Fire brigade, Mumbai Police and civic officials rushed to the site but the constricted lanes made it difficult to access the area, reduced to a mass of rubble, twisted concrete and broken wires.
Scores of locals joined in the effort, forming a human chain to help in removing the debris brick by brick and picking up slabs of concrete to locate those buried in the predominantly Muslim area.
The rescue work, hampered by narrow lanes, was further hindered by politicians, including ministers and legislators, making a beeline for the site.
Prime Minister Narendra Modi was among those who expressed his anguish at the loss of lives.
"Collapse of a building in Mumbai's Dongri is anguishing. My condolences to the families of those who lost their lives. I hope the injured recover soon. Maharashtra government, NDRF and local authorities are working on rescue operations & assisting those in need," the Prime Minister's Office tweeted, quoting Modi.
TV channels showed dramatic visuals of a child, wrapped in a cloth, being carried out of the debris by rescue workers. The child is alive, officials said. A barely conscious woman was also pulled out and carried in a makeshift stretcher made of cloth.
There were many anxious, frantic moments as family members of those missing waited for some news, their hopes dimming with each passing hour.
Mumbai Mayor Vishwanath Mahadeshwar said he had asked the municipal commissioner to launch a probe.
The BMC has opened a shelter at Imamwada Municipal Secondary Girls School after the building collapse, a civic official said.
"We are assuming that 10 to 12 families are still under the debris," Mumbadevi MLA Amin Patel told reporters at the spot.
As rescue efforts went on, there was confusion about which body owned the building.
Locals said the building belonged to the Maharashtra Housing and Area Development Authority (MHADA). However, Vinod Ghosalkar, chief of MHADA repair board, denied that was the case. Legislator Bhai Jagtap said residents had complained to housing authorities to take prompt measures as the building was very old and had long been in a dilapidated state.
Some part of the building was left standing after the collapse.
A civic official said around 500 buildings were declared as dilapidated this year but of them, only 68 have been evacuated.
According to Congress leader and long time Mumbaikar Milind Deora, the story was the same each monsoon.
"This is unfortunately something that happens in Mumbai every year during monsoon. Walls collapse, there are pot holes on roads where people die and young boys fall into manholes," he said.
The former Union minister said it was time for Mumbaikars to seek an explanation from the government for this recurring problem.
There was heavy deployment of police personnel who had barricaded the affected areas.
Mumbai Municipal Commissioner Praveen Pardeshi and Police Commissioner Sanjay Barve were among those who visited the site of the building collapse.
While Mumbai has thousands of glitzy buildings and structures, including the Mumbai international airport and the Worli Sealink, there are many more colonial era apartments, residential complexes and public infrastructure such as bridges and roads that are in dilapidated condition.
Earlier this month, over 20 people were killed in multiple incidents of wall collapses in the city's worst monsoon rains in a single day in 14 years.
In March this year, five persons were killed and 29 injured after a section of a foot overbridge outside Mumbai's Chhatrapati Shivaji Maharaj Terminus collapsed during rush hour.
And in July last year, the pedestrian section of the Gokhale bridge in Andheri bridge collapsed, killing two people.
Tragedies such as Tuesday's, especially induced by monsoon rains, are all too common in Mumbai metropolitan area, India's largest city by population with 22.5 million people.
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