A 21-year-old man and seven minors were arrested on Wednesday on charge of robbing an auto-rickshaw driver at knife-point, police said.The victim was driving from Shastri Park to Khajuri in north Delhi late at night when the eight accused boarded his auto-rickshaw and asked him to take them to Khajuri Chowk, police said. One of them threatened the driver with a knife near 5th Pusta Usmanpur while the others snatched his mobile phone and Rs 1,500, Deputy Commissioner of Police A.K. Singla said. As they fled in different directions, the auto-rickshaw driver chased one of them, which was witnessed by a police officer who caught him. Singla said main accused Ashif, 21, of Uttar Pradesh led police to the others involved in the crime.