Militants belonging to a group called Sri Ram Sena, who claim to be custodians of Indian culture, said Valentine's Day is un-Indian.
The threat comes days after the group's activists stormed a bar in the south western city of Mangalore, dragging out and beating women they accused of acting obscenely and "going astray".
The attack led to fears an extremist "Hindu Taliban" was on the rise in India.
Gangadhar Kulkarni, an activist in the group, which is a radical wing of the Hindu nationalist movement, said: "If people celebrate the day despite our warning, then we will definitely attack them."
"Valentine's Day is definitely not Indian culture. We will not allow celebration of that day in any form," added Pramod Mutalik, the group's founder.
Several Hindu radical groups aggressively attack what they perceive as the corrosive influence of Western culture on India.
As well as objecting to kissing in Bollywood films and open displays of affection they have targeted conversion away from Hinduism and marriages between people of different faiths.
The attack on the Amnesia lounge in Mangalore also reopened debate on what was acceptable behaviour for young Indian women.
While India is a traditionally conservative society which places pressure on women to behave modestly, members of the younger generation are more likely to visit bars and restaurants with their friends.

http://www.telegraph.co.uk/news/worldnews/asia/india/4447238/Hindu-extremists-will-attack-Valentines-Day-couples.html