Kathmandu, Sep 6 - More than three months after a bomb went off during mass at one of Nepal's oldest churches, killing three women -- two of them Indians, police Sunday arrested the mastermind who is the head of an outlawed Hindu militant organisation.

Ram Prasad Mainali, chief of the Nepal Defence Army, a shadowy outfit that had asked all Christians and Muslims to leave Nepal or face dire consequences, was caught with three associates from southeast Nepal.

The four men were said to be armed and were caught from Rajgarh between Jhapa and Sunsari districts in eastern Nepal.

Mainali and his NDA, that claims to be training Hindu suicide bombers, had taken responsibility for an explosion at the Assumption Church in Kathmandu Valley's Dhobighat area in May, killing a schoolgirl and two women during mass.

Soon after the blast, police had arrested Sita Thapa, who reportedly admitted to having planted the bomb in the prayer hall, after being paid by the NDA.

Though Thapa is behind bars with police having charged her with the three deaths, Mainali and the NDA had been at large, continuing to intimidate Nepal's Christian community with extortion calls and death threats.