The Catholic archbishop of Delhi has sparked an angry rebuke from the ruling Hindu nationalist party after he warned India's secular constitution was under threat.

The unprecedented political intervention comes amid accusations from a number of Christian groups that the President Modi's BJP party tolerate and even incite violence against Christians.

Archbishop Anil Couto wrote a letter to all parish priests and religious institutions in the archdiocese of Delhi that brought to the fore tensions between Christians and Hindu nationalist hardliners who form the bedrock of Modi's Bharatiya Janata Party (BJP).

'We are witnessing a turbulent political atmosphere which poses a threat to the democratic principles enshrined in our Constitution and the secular fabric of our nation,' he wrote.

Archbishop Couto called for a prayer campaign until the general election, which has to be held by next May.

But the BJP accused the archbishop of calling people to vote along communal lines. ANI television news agency quoted BJP minister Giriraj Singh as saying that 'every action has a reaction', while another BJP leader, Vinay Katiyar, said the archbishop's comments could lead to 'communal tensions'.

The spokesman for the Delhi Catholic Archdiocese, Father Savarimuthu Sankar, hit back and said the archbishop only called for a prayer campaign not an election campaign.

He pointed to continued violence against Christians over the past four years and attempts to bring back to Hinduism people who converted to Christianity.

'We try to influence at least those people who are educated, who are balanced,' said Sankar. 'So far we have been saying these are fringe elements who are behind the attacks. But there is a danger that fringe elements may become the mainstream. To some extent they are succeeding also.'

Anti-conversion laws have been passed in seven Indian states meaning anyone wishing to convert has to obtain permission from the state government and clergy have to give a month's notice before performing a conversion ceremony.

It comes after the Evangelical Fellowship Of India, an alliance of evangelical Christians in India, documented at least 351 cases of violence against Christians last year, mainly in states ruled by the BJP. It said in a report in February that 2017 was one of the most traumatic years for the Christian community in India since Hindu-Christian clashes killed more than 90 people, largely Christians, in the eastern state of Odisha in 2008.

Additional reporting by Reuters.