Far-right Hindu nationalists in Mumbai have doused the head of an Indian think-tank in black ink in protest at the launch of a book by a former Pakistani foreign minister.
The Shiv Sena party said the ink attack on Sudheendra Kulkarni was a form of "peaceful protest" against Pakistan.
The party later cancelled a planned protest at the launch of Khurshid Mahmud Kasuri's book amid an outcry.
Mr Kulkarni called the incident "an attack on democracy"
He was later admitted to hospital to have the ink removed.
The Shiv Sena party is a junior coalition partner in the Maharashtra state government, which is ruled by India's governing BJP.
The party was founded in 1966 to keep south Indian migrants out of Maharashtra state and to halt the spread of Islam.
Over time, it has acquired a reputation for promoting religious and ethnic chauvinism while allegedly targeting minorities, especially Muslims.
"A group of 10 or 15 Shiv Sainiks mobbed me, they stopped my car, asked me to come out, they caught me, started abusing me, they said we had ordered you to stop the launch this evening, you didn't listen to us, this is what we'll do with you," Mr Kulkarni told reporters.
The activist attended several press conferences with the black ink still on his face before being admitted to hospital. Khurshid Kasuri sat alongside him.
Shiv Sena called its stunt "non-violent and historic".
"The ink attack on Kulkarni is a mild reaction from [Shiv] Sena - this is not ink but the blood of our soldiers," party leader Sanjay Raut told reporters.
Later Mr Raut said the protest had been called off following a conversation with the state's chief minister, Devendra Fadnavis, who had told him that "there should be no anti-Pakistan protest at the event".
Shiv Sena's action drew widespread condemnation, including from senior BJP leader LK Advani.
"This morning I saw on TV my long-time colleague Sudheendra Kulkarni has been attacked. I do not want to name anybody, but such incidents show a growing intolerance towards views that are different from one's own. I strongly condemn the attack," he was quoted as saying by the Indian Express newspaper.
The incident comes amid a series of events that have led to growing concerns about the treatment of minorities in India.
Last month a 50-year-old Muslim man from north Indian was lynched over rumours that he had stored and consumed beef.
The Shiv Sena party also forced the cancellation of a concert by Pakistani singer Ghulam Ali last week.
And last week state lawmakers from the ruling BJP party attacked an independent Muslim politician in Kashmir because he had held a "beef party".