After four hours of emergency surgery to deal with critical injuries to his liver, lower chest, abdomen and pancreas, doctors at Hinduja Hospital said the 56-year old Mahajan was "quite critical".
Meanwhile, the police described the attack by Pravin as "pre-meditated."
Three bullets that caused extensive damage leading to massive bleeding have not yet removed from the body. Two army doctors, specialised in warfare injuries, were brought in yesterday evening to help the medical team.
Mahajan was shot at his Worli residence by his brother Praveen, 10 years younger to him, following a heated argument.
Pravin, who surrendered after the shooting and was arrested, reportedly told his interrogators that he had taken the step because his brother "ill-treated" him. Praveen's lawyer, however, claimed that he was "mentally ill."