Hindu mobs have damaged more than a dozen churches and homes, and destroyed a Christian orphanage in an arson attack which left one woman dead. The violence has so far left 11 dead, mainly Christians.
The violence was triggered by the murder of Swami Laxamanananda Saraswati, leader of radical Hindu group Vishwa Hindu Parishad (VHP), on Saturday.
Saraswati had been leading a campaign against Christians in Kandhamal district over conversions, a longstanding source of tension between Hindus and Christians in the area.
The Evangelical Fellowship of India said it had received reports that three adults and one child were killed in violence in Barakhama, one of the worst hit areas.
Town authorities have set curfews and ordered police to shoot rioters on sight in 11 towns across Kandhamal, reports Reuters news agency.
The violence has been condemned by Christian groups, including Gospel for Asia (GFA) which says that one of its missionaries was attacked whilst on his way to church on Sunday.
GFA said it had received reports of more than 75 separate attacks on GFA-related work in Orissa state.
The group's president KP Yohannan condemned Saraswati's killing, which authorities have blamed on Maoists.
"We are deeply shocked and anguished to hear of the atrocities and violence meted out to the innocent Christians and churches in Orissa as a backlash of the sorrowful event and we are praying for an immediate halt to the violence, said Yohannan.
GFA said that many pastors and their families have gone into hiding in forests, some without food and water.
"What is most disturbing is that these attacks seem to be well organised and orchestrated," Dr Yohannan said.
Archbishop Vincent M Concessao, President of the National United Christian Forum, has urged Christian schools and colleges to close on Friday "as a mark of our solidarity with our suffering brothers and sisters and as a protest against all acts of communal violence and atrocities on the Christian community and other innocent people".
The NUCF has also declared Sunday 7 September 2008 an official day of prayer and fasting.
Pope Benedict has also condemned the violence and the killing of the Hindu leader, saying, "While I firmly condemn every attack on human life, whose sacredness required respect by all, I express my spiritual closeness and solidarity to the brothers and sisters in the faith who are so sorely tried."
