Members of the Opposition disrupted a question and answer session in the Rajya Sabha for the fifth consecutive day, forcing it to be adjourned.
The issue of religious conversions has come to the fore since an affiliate of the ruling Bharatiya Janata Party announced the "re-conversion" of 57 Muslim families to Hinduism in early December.
Some claimed the families had been deceived or even forced to convert but the Hindu nationalist group responsible said there would be a larger ceremony for thousands of new Hindu converts on Dec. 25.
The leader of the Opposition, the Congress Party's Ghulam Nabi Azad, first raised the issue, saying that it was a serious matter and no one but the prime minister could give an assurance on it.
"The prime minister gets respect wherever he goes because he represents the diverse people of India. When there were doubts about his image, he was not welcomed in foreign lands," Azad said in Parliament.
"He gave them pain, now he will also give them the remedy," he said, adding that Modi should give confidence to Muslims, Sikhs, Christians and other minorities that they are "not just safe in the country but also that their religion is safe."
The house was adjourned for two and half hours after opposition members insisted on a reply from Modi.
The government's stand meanwhile was that it is ready for a discussion and that Home Minister Rajnath Singh will issue a response.
Modi made an appearance in the Upper House on Dec. 18 but did not speak on the issue despite repeatedly questions from members of the house.
A right wing Hindu organisation, the Dharm Jagran Samiti, had "re-converted" about 150 Muslims to Hinduism in the city of Agra on December 8.
“We are aware of recent reports regarding alleged coercion in mass conversions in India, and are monitoring the situation closely. We work to promote respect for freedom of religion and belief in all countries around the world. We call on all states to comply with their obligations to protect religious freedom, as per international norms,” said a statement issued by the US Department of State.