Just after 1.30pm on Friday, the loudspeaker outside Sarai Alawardi mosque crackled to life, and more than a thousand foreheads were touched to the hessian mats that lined the ground. Towering over them were the skyscrapers of Gurgaon, a satellite town south of Delhi that houses technology companies, bowling alleys and other symbols of the “new India”.

A day after the Hindu nationalist prime minister, Narendra Modi, claimed a landslide election victory, some in the congregation were anxious about whether this new country had a place for them. “These days, it isn’t safe for us here any more,” said Haji Shezhad Khan, the chairman of a local Muslim activist group, sitting in a shaded courtyard a few metres from the mosque.

For many Indian Muslims – whose population of about 200 million would comprise the seventh-largest country on earth – Modi’s emphatic re-election has been an isolating experience.

The country’s most acrimonious election campaign in recent history was studded with references to unauthorised migrants from Bangladesh as “termites”, the nomination to parliament of a Hindu accused of terrorism and a debate over whether Mahatma Gandhi’s assassin – who killed the founding father for supposedly cowing to Muslim demands – was in fact a patriot.

Despite this, or perhaps because of it, a record 270 million Indians cast their votes for Modi’s Bharatiya Janata party (BJP) or its allies.

“We truly believed it would be fought back,” said Nazia Erum, an author who has written a book about raising a Muslim child in today’s India. “We believed that a lot of voting that happened in 2014 was based on Modi’s development agenda and people would be able to see through it now and things would be different. And as it turns out we were entirely mistaken.”

Friction between Hindus and Muslims, as well as tension among sects within both faiths, has been a persistent feature of Indian life. But in the past five years violence against Muslims has increased, including at least 36 killings by “cow vigilantes” of cattle farmers and traders accused – usually spuriously – of harming the revered animals.

In Gurgaon, where hundreds of thousands of Muslim migrants have arrived in the past few years along with Hindus to work in factories and on construction sites, tension has been boiling over. Bitter campaigns have been waged against Muslims praying in public spaces because mosques have no capacity or are too far away. Sanctioned prayer spaces have been gradually whittled down to just over three dozen after protests by Hindu organisations. “They are not allowing us to pray,” said Khan.

Rajeev Mittal, the head of a Hindu nationalist group that has campaigned against mosques in the area, insists his campaign is strictly about upholding municipal planning laws. “We are not against people offering prayer, but it should be done in the mosque or in all the areas designated for them,” he said.

The BJP points to statistics that show there have been no large-scale religious riots under Modi’s prime ministership, and no surge in bias crimes in the country’s official data – though some rights groups argue this information is patchy and unreliable.

The impact of Modi’s rule has been to embolden extremists, his critics say, and create a culture where religious chauvinism and impunity can flourish.

“More than riots, Muslims fear the pinpricks,” said Meenakshi Ganguly, the south Asia director for Human Rights Watch. “It’s the Muslim vegetable vendor who is suddenly beaten up, it’s when Muslim families say they are worried about taking lunch boxes because they don’t know when they’re going to be accused of carrying beef. “People feel entitled to impose their voices, and to do so violently, and there is no assurance the state will step in and protect them.”

Modi’s supporters and opponents alike recognise that his victory on 23 May is the cementing of an ideological shift in what will soon be the world’s most populous country. Most elections are a choice between competing visions, but India’s polls this year were, in the words of the Congress MP Shashi Tharoor, “a battle for India’s soul”.

In dispute is a century-old argument about the myths that should fuel Indian nationalism. The country’s first prime minister, Jawaharlal Nehru, surveyed the extraordinarily diverse subcontinent and conceived it as a parchment “on which layer upon layer of thought and reverie had been inscribed, and yet no succeeding layer had completely hidden or erased what had been written previously”.

Opposing him were Hindu nationalists such as Vinayak Savarkar, an atheist but one who viewed Hinduism in its innumerable manifestations as a set of cultural practices that bound the subcontinent’s people together as a single nation. His vision left little room for Muslims or other minorities.

“Mohammedan or Christian countrymen … are not and cannot be recognised as Hindus,” Savarkar wrote in a 1923 treatise. “Their holy land is far off in Arabia or Palestine. Their mythology and god men, ideas and heroes are not children of this soil.”

The modern Hindu nationalist movement has evolved from Savarkar’s views, said Rajat Sethi, a fellow at the India Foundation, a think-tank aligned with the right-wing Hindu umbrella group, the Rashtriya Swayamsevak Sangh (RSS), of which Modi is a lifelong member.

“Savarkar talks about a more militarised Hinduism … The RSS would say, no, it’s about culture,” Sethi said. “Hinduism is a community based on shared culture practices rather than a dogmatic book.” In this way, he said, Muslims and Christians were also Hindus: their lifestyles and rituals also inflected by India’s Hindu civilisation. “Muslims form an integral part [of the nation] because a lot of what we stand for is incomplete without Muslims as a religion.”

The ostensibly “secular” politics of Nehru’s Congress was really a byword for courting Muslim votes by giving the community special privileges, he added, such as political autonomy for Kashmir, and the right to govern marriages and other social affairs according to Islamic law – both of which Hindu nationalist groups target for reform.

Nehru’s vision now appears to be in terminal decline. The Hindu nationalism he tried to sideline, including by banning the RSS, has been granted a clear popular endorsement.

Its worst excesses may be borne by the poor, but wealth and privilege are no shield, said Erum, who researched her book by interviewing more than a hundred children and their parents at some of the most elite schools.

“It’s happening in classrooms, in playgrounds: kids are bullied on religious lines, they are reflecting the fractures in our society,” she said. “It’s happening in the best schools, the most metropolitan cities. This is no longer the fringe.”

She blames in part the country’s 24-hour news channels, which, along with social media, fixate on divisive issues that draw eyeballs but promote a vision of a country in perpetual argument. “It is an unending culture war,” she said.

“Growing up in India was one of the best experiences,” she added. “Religion was not a factor you considered when you played or shared tiffins. But now it is.”