A 92-year-old woman was kicked in the head and left badly injured on her doorstep in north Manchester.
Edith Brickhill had opened the door to let her dog out and was bending down when she was set upon by an attacker on Wednesday night.
Mrs Brickhill suffered severe bruising, two large lumps to her forehead and a cut nose. Police said she was left extremely shaken up and distressed.
A man was arrested on suspicion of assault and remains in custody.
Det Insp Denise Sweeney, of Greater Manchester Police, said: "At this early stage, we are still investigating what motivated such a senseless attack on a defenceless, vulnerable pensioner as she was letting her dog out.
"Whatever the motive, it was an act of sheer cruelty and we are doing everything we can to get to the bottom of it.
"We have a team of officers, both detectives and neighbourhood officers, who will be investigating this awful crime."
Officers are increasing patrols in the area to reassure residents and anyone with concerns should approach them, the force said.
