Reformist monk Maduluwawe Sobhitha said given an opportunity, he is ready to fight Rajapaksa as the common opposition candidate.
"I will come forward if there are no other common candidate aspirants," Sobhitha said.
He pledged to abolish the present executive presidential system of governance.
68-year-old Rajapaksa, who commands considerable support from the island's Buddhist majority, is reportedly mulling snap polls during the first quarter of 2015, a year ahead of schedule.
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Sobhitha's declaration came as the nationalist Buddhist group BBS or the Forces of Buddhist Power faced criticism for its role in an anti-Muslim riot in the muslim dominates southwestern area over the weekend.
The BBS mobs were accused of beating up Muslims and setting fire to Muslim owned properties.
In a related incident, a Buddhist monk who attempted to form a rival organisation against the BBS was assaulted and dumped on a roadside this morning, the police said.
Watareka Vijitha, the monk was accused by the BBS of siding with the Muslim minority in his bid to form an organisation named the Nationalist Force.
The BBS activists stormed his press conference and forced him to withdraw his movement.