The chairman of the hardline faction of the Hurriyat Conference Syed Ali Shah Geelani has warned of a backlash if the BJP came to power in the state.
He said it will turn Kashmir into "a laboratory for the ghar wapsi (conversion to Hindusim) campaign of the "fascist forces".
Geelani's statement, his first since the marathon assembly polls threw up a hung House, comes as BJP leaders are engaged in informal talks with the single largest People's Democratic Party (PDP) to form a coalition government.
Without naming the PDP, Geelani said "communal forces in India are being aided by enemies of Muslims in the state''.
Geelani, who termed the current political situation in the state as "alarming", said a BJP government will erode the state's Muslim identity, culture and also its resistance movement.
"People should remain mentally prepared for a decisive resistance," he said.
The separatist leader also said mainstream political parties in Kashmir have failed to raise their voices against "communal designs".
"These people (mainstream political parties) are power hungry and their first and last goal is political benefit. Some of them have cooperated with these communal forces in the past and some are in line to do so today," he said.
"At this crucial juncture if any type of carelessness or leniency is shown, it will have serious consequences and we will lose our existence forever," Geelani added.
The PDP has been negotiating with the BJP for weeks now, although both parties have denied formal dialogue.
Although the saffron party failed to open its account in the Valley, it won 25 seats, mostly in the Hindu heartland of Jammu region.
Unlike in the past, Geelani had given a muted call for election boycott which observers said was aimed at keeping away the BJP, which was banking on boycott so as to leverage migrant votes in the Valley.