NEW DELHI — The police on Wednesday detained a member of Sanatan Sanstha, a right-wing Hindu organization, in the killing last week of an activist who often debunked village mystics and had campaigned for a law banning “black magic.”

The police detained the man, Sandeep Shinde, at the organization’s headquarters in the coastal state of Goa, said Rajesh Bansode, the deputy police commissioner in Pune, where the killing took place. Mr. Shinde will be questioned by investigators in Pune, he said.

The killing of the activist, Narendra Dabholkar, who had delivered lectures to generations of schoolchildren, prompted a wave of emotion last week. The governor of Maharashtra State, where Pune is located, signed the so-called black magic act into force as an ordinance, and police officials had come under intense pressure to crack the case.

Associates of Dr. Dabholkar, a nonpracticing physician, said that over the years he had received threats from right-wing organizations, and that hecklers sometimes interrupted his lectures, on one occasion splashing his face with black ink.

A spokesman for Sanatan Sanstha, a religious group active in the region, said the police began questioning its members immediately and had a list of 70 members they had planned to interview. The spokesman, Shambhu S. Gaware, said the organization viewed Dr. Dabholkar as an ideological opponent but “always chose legal means to oppose him, legal and peaceful means.”