A young man was seen slapping, kicking, punching and pulling down lawyer Prashant Bhushan from his chair.
Authorities identified the attacker as Inder Verma, whose group claimed to be upset over remarks Bhushan made about Indian-administered Kashmir.
New Delhi's police spokesman Rajan Bhagat told CNN that two other assailants fled the scene after the assault.
Bhushan told the TV station interviewing him that his attackers were angry over his comments supportive of a referendum in the Muslim-majority Himalayan territory under Indian control.
A military line divides Kashmir between Hindu-majority India and Islamic Pakistan. Both arch-rival neighbors claim the region in its entirety.
Hazare condemned the attack on his fellow campaigner.
"I strongly condemn it. We have a lot of hopes on the young people... I pray to God to give wisdom to those who take the law into their own hands," he told a news conference.
Leaders of both India's ruling Congress party and the Hindu nationalist opposition denounced the beating of Bhushan, seen as one of the legal brains behind Hazare's anti-graft movement.
Hazare's two hunger-strikes this year sparked public outrage against India's federal government over allegations of massive corruption.
The 74-year-old activist, who wears only a simple garb of homespun cotton, took vows of chastity and public service after he retired from the Indian army decades ago.
According to public statements in June, Hazare has $1,500 in his bank account.
Yet, the ascetic social activist rattled the country's leadership at the highest levels, when his fasts demanding tougher legislation to deal with corruption garnered support that cut across economic and social divides.