The incident happened in Uxbridge Road after 10.30am on Thursday December 8.
Emergency services including the Metropolitan Police and the London Ambulance Service were called to the scene at around 10.39am.
In a statement, the Met Police said a woman had been in collision with a bus, but it did not have an update on her condition as yet.
She was described as conscious at the scene.
Store manager of Greenford Timber and Builders Merchants, who requested to stay anonymous, said: "There was a lot of blood and I think there must have been a really bad head injury
"We think she was elderly, she was facing down on the road and there was blood coming from her face and her head.
"She was maybe in her 60s, maybe a bit more, she was elderly. She had a walking stick.
"At the moment police are still outside and they have taped off the road. "
One eye witness said: "I just came to the shop two minutes ago. Outside people are gathering. She's in a pool of blood".
Another person who wanted to remain anonymous said: "They've cordoned it all off.
"She's been in an ambulance and gone now."
A London Ambulance Service spokesman said: “We were called at 10.36am today to reports of a road traffic collision at Uxbridge Road, W12.
“We sent an ambulance crew and an emergency responder to the scene. T
"he first of our medics arrived at the in scene under four minutes.
“We treated a patient at the scene for a head injury and took them as a priority to a major trauma centre.”
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