Patel (86), a Harvard University alumnus, is survived by his wife and two daughters.
He died around 4am Saturday, people close to his family said, adding that he had been recently discharged from a private hospital where he was admitted for age-related ailments.
After completing a Master's in Law from Harvard University, Patel taught at a law college before starting practice in the Gujarat High Court where he championed the cause of the marginalised and filed hundreds of PILs on issues concerning the poor, Dalits, farmers and labourers.
Among his notable PILs was the one demanding minimum wages for sugarcane farmers of Surat.
Patel was also involved in the Narmada Bachao Andolan and fought for the rights of the tribals displaced by the Sardar Sarovar Narmada Dam project.
Patel fought cases for those displaced by various government projects, including the Sabarmati Riverfront.
He was selected in 1972 as a member of the Gujarat State Law Commission for a period of three years.
He actively participated in the state's 1974 Navnirman Andolan against corruption and economic crisis.