* India's Hindu activists grow increasingly bold
* Some seek to convert non-Hindus, raising concern
* Minorities and moderates worried by developments
* Hindu nationalist Modi swept to power in May
By Rupam Jain Nair and Frank Jack Daniel
HASAYAN, India, Sept 5 (Reuters) - Fired up and full of vitriol, Hindu activist Rajeshwar Singh is on a mission to end centuries of religious diversity in India, one conversion at a time.
His voice echoing off the walls of a Protestant church across a narrow street, Singh railed against foreign faiths at an event last week to convert a Christian family to Hinduism in the rural town of Hasayan, 140 km (87 miles) south of Delhi.
"We will cleanse our Hindu society. We will not let the conspiracy of church or mosque succeed in Bharat (India)," he said, standing in the family's front yard by a ritual fire lit to purify the poor, lower-caste converts.
Emboldened by Prime Minister Narendra Modi's rise to power in May, leaders of his Hindu nationalist Bharatiya Janata Party (BJP) have joined right-wing activists like Singh to openly declare India a nation of Hindus, posing a challenge to its multi-faith constitutional commitment.
About a fifth of India's 1.27 billion people identify themselves as belonging to faiths other than Hinduism.
Singh is affiliated to the Rashtriya Swayamsevak Sangh (RSS), a vast nationalist volunteer organisation that aims to unify Hindus "to carry the nation to the pinnacle of glory".
The RSS brought Modi into politics as a young man and its foot soldiers helped cement his May election victory in India's heartland, most notably in the country's most populous state of Uttar Pradesh, where Hasayan is located.
The RSS has grown in prominence since the general election, with members appointed to key cabinet posts and senior leaders deputed to the party.
Increasingly hardline statements by RSS chief Mohan Bhagwat, an old friend of Modi, have helped motivate millions of volunteers, like Singh, already excited by the prime minister's May victory.
"Just as those who stay in England are English, those who stay in Germany are German, and those in U.S. are Americans, all those who stay in Hindustan are Hindus," Bhagwat said in August, angering India's Muslim and Christian minorities.
The debate triggered by the comments revealed a deep ideological rift between those who believe the term describes a national identity as well as a religion, and liberals who think in a multi-faith nation, all cannot be called Hindus.
Adding to the controversy, RSS-linked groups have stepped up a campaign against "Love Jihad" - a term for what they consider to be an Islamist strategy to convert Hindu women through seduction, marriage and money.
Their fears about Islam may be fuelled by al Qaeda leader Ayman al-Zawahri's announcement this week of the formation of an Indian branch of his militant group.
Previous police investigations have found no evidence of an organised "Love Jihad". But the concept has gained credence across central India in recent weeks, leading to sometimes- violent protests, despite being considered an absurd conspiracy theory by mainstream, moderate Indians.
While avoiding the term "Love Jihad", Modi's BJP last week adopted the subject of forced conversions as a campaign issue ahead of Sept. 13 by-elections in Uttar Pradesh, a state prone to sectarian strife.
Simultaneously, activists like Singh have stepped up what they see as necessary defensive measures - converting others "back" to Hinduism. Hinduism is not normally considered a religion that seeks converts, but it does not have strict rules against the practice.
"The Hindu wave has just begun. In 10 years we will convert all Christians and Muslims," shaven-headed Singh said with a grin after Friday's conversion ceremony, to murmurs of approval from other organisers of the ritual.
His colleagues included a former Adventist preacher now dedicated to Hindu "homecoming" conversions and a businessman from the city of Agra, home to the world-famous Muslim-built monument, the Taj Mahal.
"The BJP is our political organisation. They are our brothers. We have ensured that they won the election. Modi is a Hindu leader," Singh said. "This is our golden age."
Singh's 10-year deadline is unrealistic in a country of 175 million Muslims, who account for around 15 percent of Indians and constitute the third-largest Muslim population in the world, as well as other faiths.
But such displays of bravado are worrying moderates in a country whose long history of inter-religious co-existence is punctuated by bloody outbreaks of strife.
Last year Hindu-Muslim riots left 65 dead just 140 miles (90 km) from Hasayan. Tensions are still high across Uttar Pradesh, which is governed by a party many consider to unduly favour Muslims.
In 2002, riots also broke out in Modi's home state of Gujarat, just after he was elected chief minister there. More than 1,000 died, mostly Muslims. Critics say he did too little to stop the violence, but a Supreme Court investigation found no evidence to prosecute him and he denies any wrongdoing.
During this year's general election in India, in which Modi elsewhere focused on development, his campaign in Uttar Pradesh stood out by exploiting anti-minority feeling to unite low- and high-caste Hindus into a voting bloc.
Uday Vir from a "Dalit" low caste was at the centre of the reconversion ceremony last week, conducted by high-caste Hindus.
Born into a Christian family, Vir, 55, said a land dispute with the church was the reason he was switching religion, before chipping off a black stucco cross from his porch with a hammer.
Tensions such actions trigger were clearly visible on the Hasayan side street. As tempers frayed briefly under the midday sun, a burly Hindu activist accused a Christian priest of luring people to convert with money and of keeping women and children locked inside the church's faded red walls.
"They are lying. I am not here to convert anyone. My job is to manage the church," said Reverend Dinesh Kumar as a dozen policemen struggled to control the swelling crowd.
Ahead of the by-elections this month, a prominent BJP member of parliament from the state, Yogi Adityanath, has been accused of delivering inflammatory speeches.
In one video he asks supporters to convert 100 Muslim women through marriage every time a Muslim man marries a Hindu. In another, he said religious riots happen wherever more than 10 percent of the population is Muslim.
Adityanath, also a senior Hindu priest, has not been reined in by his party and, instead, features as a star campaigner in the by-election race. He says the footage was doctored.
NO EVIDENCE OF CONSPIRACY
Earlier this month a national-level rifle shooter Tara Shahdeo complained of being a "Love Jihad" victim, saying her Muslim husband hid his religion when they married in the eastern city of Ranchi.
In response, hardline Hindu outfits rampaged through the town and declared a day-long strike. Other violent protests have broken out in recent days close to New Delhi.
A pamphlet named "Love Jihad" is being widely circulated by members of RSS at Hindu weddings, festivals and outside colleges across the country.
Written in 2011, it links the concept of "Love Jihad" to the rule of Muslim Mughals in India centuries ago - a popular theme with Hindu nationalists who feel Hinduism was weakened by foreign rule.
Police say sporadic cases of trickery by unscrupulous men are not evidence of a broader conspiracy. In Uttar Pradesh, police found no evidence of attempted or forced conversion in five of six reported "Love Jihad" cases in the past three months.
"In most cases we found that a Hindu girl and Muslim boy were in love and had married against their parents' will," state police chief A.L. Banerjee told Reuters. "These are cases of love marriages and not Love Jihad." (Additional reporting by Jitendra Dash in Bhubaneswar and Sharat Pradhan in Lucknow; Editing by Mike Collett-White)