Prohibitory orders were on Sunday clamped in communally-sensitive Kandhmal district and security tightened across Orissa after the killing of a prominent Vishwa Hindu Parishad leader and his five associates sparked protests in the state with a prayer house being torched.

Activists of VHP and other saffron outfits staged demonstrations, blocked roads and stopped trains at several places since Saturday night after the 85-year-old VHP leader, Laxmanananda Saraswati, who had been working among tribals against religious conversion, was gunned down at an ashram for girls at Jalespeta in Tumudibandh block.

The Hindu outfits have given a call for 12-hour Orissa bandh on Monday.

The situation in the tribal dominated Kandhmal district is tense but under control" after the killing of VHP activists last night by unidentified attackers, Kandhmal District Collector Krishan Kumar told PTI over phone.

Prohibitory orders under Section 144 CrPC were clamped in the district to prevent any flare up, he said.

A small thatched structure used as a prayer house was set afire by unidentified persons late on Saturday night at Tentulijhari in Sundergarh district, Inspector General of Police (Western Range) Pranabindu Acharya said.

Nobody was injured in the incident, he said adding, efforts are on to nab the miscreants.

"We are keeping a close watch since the incident took place," Kumar said.

A high alert was sounded across the state and additional security forces sought from the Centre.