A woman suffered serious facial injuries when a corrosive substance was thrown at her, police said.
The 47-year-old was attacked by someone on a passing moped or pushbike in the area of Northbrook Street, Ladywood, Birmingham at about 10:00 BST.
West Midlands Police said they believe it was a "deliberate" attack and the victim's injuries "look to be very severe".
The woman is receiving specialist burns treatment, ambulance staff said.
Police said it is thought a member of the public transported the victim to Astbury Avenue in Smethwick where an ambulance crew attended to her.
The victim's daughter told the BBC that her mother was in a critical condition in hospital, and she feared she may lose her eyesight.
A West Midlands Ambulance Service spokeswoman said: "The woman was given pain relief to help ease her discomfort whilst her face was fully irrigated with water to help remove the unknown substance.
"She was then transferred by land ambulance on blue lights to hospital for further specialist burns treatment."
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Astbury Avenue resident Angie Whitehouse said: "I [saw] a small ambulance to the left of my house and a large one to the right, and I took no notice and I didn't know what had happened.
"Then a few hours later we had [a] door-to-door PCSO come to the house. She said an incident had happened in Birmingham and the lady was OK, but she'd been taken to hospital and she had been down this street to her friends."