Nagpur, June 1 (IANS) Three terrorists wearing police uniforms and armed with AK-47 rifles and grenades were shot dead when they tried to barge into the headquarters of the Rashtriya Swayamsewak Sangh (RSS), the pre-eminent Hindu nationalist organisation, early Thursday, police said.

Three police personnel, including a sub-inspector, were injured in the shootout, police sources said. The terrorists were trying to enter from the rear side of the multi-storey RSS building in old Nagpur at about 4.15 a.m. when they were detected by security personnel who opened fire on them.

No organisation has claimed responsibility for the attack so far but police say the modus operandi and the planning point to the hand of the Lashker-e-Taiba, the terror outfit that is based in Pakistan-administered Kashmir.

This attack comes within days of a series of arrests of suspected militants, shootouts and seizures of explosive materials from terrorists in the Maharashtra cities of Aurangabad and Nashik.

Police sources said in Mumbai that the RSS top brass - none of whom were inside the building at the time - were warned of such an attack in the wake of intelligence reports of suspicious militant movement in and around Nagpur.

The attack was probably timed to coincide with the morning 'shakhas', or drill, that is held at around 5.30 a.m. and is attended by large number of Hindu volunteers.

The RSS is the ideological parent of the Bharatiya Janata Party (BJP) and right-wing Hindu nationalist outfits that go under the banner of the Sangh Parivar.

In New Delhi, Prime Minister Manmohan Singh condemned the attack and said "the entire nation remains united in its fight against terrorism" and appealed to the people to remain calm and maintain communal harmony.

RSS spokesman Ram Madhav said "Hindu centres have been a major target for the last few months. They (terrorists) have been emboldened to the extent to attack the RSS headquarters today. It is a matter of major concern for us. We want the government to ensure safety of all these places."

Maharashtra Chief Minister Vilasrao Deshmukh said security forces had "defused" the attack plan and a team is on the job. We have an anti-terrorist squad in Mumbai and we are considering having similar squads at Nagpur and Aurangabad and, if needed, other places as well."

Former prime minister Atal Bihari Vajpayee, who is on a holiday in Himachal Pradesh, also appealed for calm and said the situation should not lead to lawlessness.