The incident took place in a corridor of the court building at around 1 pm when the accused had been brought from Arthur Road jail for a hearing.
Uday Shankar Pathak (32) attacked Kalpesh Patel (28), a co-accused who has turned approver (prosecution witness), with a sharp piece of metal, police said.
"Kalpesh escaped with minor injuries and was taken to hospital. He was discharged after treatment," said Dr Manojkumar Sharma, deputy commissioner of police (zone-I).
Pathak and others are accused of killing four persons, residents of Shantarampada in Kurar in suburban Mumbai, in June 2011 after abducting them over a minor tiff in a bar.
A total of nine persons were arrested in the case.