The bleeding child was brought to a hospital on Sunday where she had to undergo a surgery and her condition is stable now, news agency Press Trust of India reported, quoting sources.
The man, who is reportedly the first cousin of the child and lives in the same building, has confessed to the crime, reported PTI.
Referring to the case, Delhi Women's Commission chief Swati Maliwal, who went to the hospital to check on the child on Monday, tweeted:
Police said the baby, whose father works as a labourer, was alone at her home in West Delhi's Subhash Nagar on Sunday afternoon. When her mother, a domestic help, returned from work, she found the child covered in blood and crying.
What to do? How can Delhi sleep today when 8 month baby has been brutally raped in Capital? Have we become so insensitive or we have simply accepted this as our fate? Where is the sena which attacked a school bus to 'protect honour'? No one to ask questions from the system now?Swati Jai Hind (@SwatiJaiHind) 29 January 2018
Her sister-in-law, who was supposed to baby-sit, said the child's cousin was playing with her for a while.
The man, who had gone missing, was arrested later from the neighbourhood.