However, other residents of the South London street where it fell say it had not yet been switched on, adding they feared it was not correctly fitted.
Clear Channel, the advertising company that installed the board, is now investigating the 39 other digital signs that were rolled out towards the end of last year over concerns of their safety.
People ran across the street and from houses to comfort the woman as she lay trapped on the floor in the minutes before the emergency services arrived.
Bricks and cement also rained down on the seriously injured victim, who remained conscious throughout, and was then rushed to the nearby St George's Hospital.
Local property developer Paul Higgins, 51, raised doubts about the sign's installation which he believes was completed on Monday (UK time), the day before it plummeted to the pavement.
Mr Higgins said the billboard's fixing bolts appeared to be too short.
'Look how heavy it is. It must be a few tonnes. There's something terribly wrong', he said.
Peace Clague, a mother-of-three who lives above the adjacent shop, said: 'The whole building shook and it sounded like an earthquake. My husband heard a scream and ran out there.
'We were really worried about our son who had just left for school.'
He said she saw the woman trapped at her waist by the sign, which allowed her to call 99 herself.
She said: 'Thank god for the way it fell. It's a miracle. The sign landed on her hips. She was still talking and was on her phone. I think she rang the ambulance and the police.'
The billboard's owners Clear Channel say they are investigating what happened, but residents have claimed the sign was only installed a few day ago and believe the wall could not hold its weight.
The woman's condition has not been revealed, although she was conscious after the incident and witnesses said she is 'lucky to be alive' having suffered injuries to her legs and hips.
Kim Yaha told the Evening Standard: 'There was a thunderous noise and I saw the billboard falling and people running towards the area.
'It just sort of dropped on the pavement, and there was debris and I saw people crossing the road to check out what was happening. It was a side street leading to a main road which was busy as you'd expect.'
A number of ambulances, as well as a fire crew, responded to the incident, but it is believed she was stuck under the board for some time.
One witness, Siobhan Slattery, tweeted: 'Thoughts are with the lady currently stuck under a billboard in Tooting- fell 15/20ft onto her. Awful.'
Explaining what happened she said: 'I was in my house and felt the whole flat shudder at about 7:35am. I looked out of the window and people started to run over to the incident.
'I went out and a lady was trapped underneath. Nothing seems to have been on her head, but the billboard was on her. The board was across her body but she appeared awake and moving her head'.
A shop volunteer at Age UK, which is blow the sign, has said it is a 'miracle' that the woman who was crushed by a freshly-installed billboard is still alive.
He said the board was only installed by advertising company Clear Channel on Sunday, replacing a wooden advertising hoarding which had been in place for some years.
A shop volunteer said: 'It's a miracle no-one died.'
Two metal bollards have been forced to the floor by the sign while a street sign sticks out at 45 degrees from underneath it. Bricks and rubble litter the street surrounding the huge digital advertising board.
Police have cordoned off the area while inspectors from Clear Channel survey the scene.
Abi Kasipillai witnessed the aftermath of the collapsed billboard.
She said: 'I just heard the billboard had fallen on a lady when I walked past.'
A London Ambulance Service spokeswoman said: 'We were called at 7.33am to reports of an incident on Mellison Road at the junction of Mitcham Road.
'We sent a number of resources to the scene including a responder by car, two duty station officers and our hazardous area response team.
'We treated a woman reported to be in her thirties for leg and hip injuries.
'She was taken to St George's Hospital as a priority.'
It is not yet known why the billboard fell off the side of the building, but owners Clear Channel say they are investigating.
A spokesman said: 'Our thoughts are with the woman who was injured.
'Our staff are attending the scene and we have launched a thorough investigation into the incident'.
The firm describes itself as one of the 'world's leading out of home advertising companies' on its website.
- Daily Mail
