A 30-year-old St Albans woman has been charged following a horrific attack in a suburban street in Melbourne's west which left an elderly woman fighting for her life.

It is believed the 77-year-old woman was out walking her dogs on Thursday evening when she was bashed and left with life-threatening head injuries on the driveway of a Willaton Street house in St Albans.

On Saturday afternoon the woman remained in hospital in a critical condition.

The 30-year-old St Albans woman was arrested on Friday morning.

On Saturday homicide squad detectives charged her with intentionally causing serious injury. She was expected to appear at Melbourne Magistrates Court. 

The 77-year-old St Albans woman was located by a passerby in a driveway about 7pm on Thursday.

Paramedics were called to the scene at about 7.35pm and she was rushed to hospital.

Hours later, a woman was arrested at a house not far from where the victim was found.

The woman smiled at television cameras as she was arrested and led to a police car on Friday morning.

She matched the description of a person wearing pink clothes and no shoes who had been spotted behaving erratically in the area about the time of the assault.

But police are searching for the passerby who went to the woman's aid. He is described as being of Middle Eastern appearance, aged in his late-40s to early 50s with a medium build and dark short hair. He spoke limited English and was wearing casual clothing, including a dark jacket.

Homicide squad Detective Inspector Tim Day described the victim's injuries as "significant".

"For that to happen a significant altercation would have to have taken place," he said.

Willaton Street resident Michael Tran heard screaming outside his home minutes before the victim was found.

"I heard a woman screaming. It wasn't normal, I knew it was wrong."

When he ran outside he saw the woman in pink darting across the road.

"She was behaving very strangely, psychotic strangely" he said.

Mr Tran said the woman ignored his questions about the source of the screaming and walked away.

He went back inside and was only alerted to the injured woman lying next door when a man of Middle Eastern appearance knocked on his door asking him to help.

Mr Tran said they found the woman bleeding heavily from her head and conscious but unable to speak.

"I asked her in Vietnamese, have you been attacked? She nodded," he said.

The woman was found outside Mandy Kaur's home in Willaton Street.

A teary Ms Kaur said she did not see the attack nor know the victim, but she got out of her shower to see a crowd of people around the injured woman in her driveway.

"It's really scary, I felt like I would fall down," she said.

"I saw her face full with the bleeding and her eyes swelling.

"She didn't speak, she didn't open her eyes."

Ms Kaur said her neighbours had called an ambulance and police. She said she was shocked it happened in her neighbourhood.

"It's pretty safe. This is a first."

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.