On Sunday, he appeared at a rally against a group of protesters (most of them women) who were objecting to a new citizenship law widely seen as discriminatory toward Muslims. There he vented his anger in a fiery speech in which he issued an ultimatum to the police: either clear out the demonstrators, who were blocking a main road, or he and his followers would do it themselves.

Image Kapil Mishra addressed a rally in New Delhi last year. Credit... Sonu Mehta/Hindustan Times

Within hours, the worst Hindu-Muslim violence in India in years was exploding. Gangs of Hindus and Muslims fought each other with swords and bats, shops burst into flames, chunks of bricks sailed through the air, and mobs rained blows on cornered men.

[Update: Violence continues in New Delhi, and the police are criticized.]

Many Indians, including Hindus, believe that Mr. Mishra and his Hindu nationalist supporters have weaponized a very dangerous mood. In a Hindu majority nation, with a Hindu nationalist government that has allowed the killers of Muslims to go unpunished, fear has been growing that violent Hindu extremism could spin out of control.

“Kapil Mishra should be in jail,” said Rupesh Bathla, a businessman who says he has known Mr. Mishra since they were teenagers. “He started communal riots. He planted hatred in other people’s hearts.”

By Wednesday, at least 25 people had died, hospital officials said, most from gunshot wounds. Several witnesses said that the live fire came from the direction of the police officers, and the dead included Hindus as well as Muslims.