It was a protest that spawned a movement. Fed up with moral policing by radical Hindu groups who had attacked women in pubs and targeted unmarried couples celebrating Valentine's Day, a group of women decided to fight back – with pink knickers.

The result was a remarkably successful dirty dissent. Not only did 40,000 items of rose-tinted underwear, much of it apparently unwashed, end up flooding the offices of the Sri Ram Sena (Lord Ram's Army), the non-violent act defused much of the tension surrounding the acts of vigilantism by self-appointed guardians of "Indian culture".

Spearheading the new movement is "A Consortium of Pub-going, Loose and Forward Women", a group of young female journalists, lawyers and academics, who began with a Facebook group protesting at the attacks on several young women last month in a pub in Mangalore, a university town in the southern state of Karnataka. Yesterday it had nearly 50,000 members.

The debate comes at a time of tumultuous social change in India, where a new generation of Indian women are experiencing new freedoms such as dating and drinking while navigating ingrained conservative values about what is acceptable behaviour.

Two events gave the clash between liberal and conservative India a deadly and sinister air. The first was the suicide this month by a 16-year-old girl who had been publicly humiliated by Hindu groups for being on a bus with a Muslim boy. The second was the threat by the Sri Ram Sena to forcibly marry unwed couples found together on Valentine's Day.

"I think we saw physical assault and criminal conspiracy as a step too far," said Mihira Sood, a 25-year-old lawyer who was part of the "core group" of the consortium. She said the reason that the pink chaddi – Hindi slang for knickers – was chosen was that it was an "irreverent" riposte to rightwingers who claimed they were saving "women from shame and dishonour".

"We wanted something irreverent and the chaddi is something we all wear but no one is supposed to talk about in Indian culture. It symbolised the hypocrisy of [the Sena's] arguments," she said.

Sood says the group may now launch a number of legal actions, and approach the country's supreme court with a petition to strike out "outdated" obscenity laws which. "Obscene acts" in public places – which are defined as causing annoyance to others – can be punished by three years in prison.

"A lot of these [laws] are used to harass women and they are so vaguely worded. They have not be changed since the mid-19th century when the Brits ran India. It is time they were struck out of the Indian penal code," she said.

On the other side of the ideological divide, the Sri Ram Sena is licking its wounds. Its "chief mentor", Pramod Mutalik, was arrested on Valentine's Day and government ministers publicly rebuked him for wanting to "Talibanise" the country.

However the Sri Ram Sena say they will not stop saving Indian culture. "We say we use clinical violence which will have a healing effect on society," said Vinay Singh, national secretary of the SRS. "We welcome any legal challenges. We say Valentine's Day is just propaganda for the market. We say pub culture is ruining our country. Let them send us chaddis. We will wear them."

Social commentators say that what the country is seeing is the first of a series of confrontations between old and new cultures. "The divide is generational. Parents do not want to know their daughter drinks or sees guys. Daughters do not want to tell their parents what they are up to. But this cannot last," said Palash Mehrotra, a columnist for Mail Today. "Young women's attitudes have changed too much in India for the clock to be turned back."