It was early morning when Achintya Das, a 55-year-old teacher in the city of Cumilla in Bangladesh, was woken by the ringing of his mobile phone. On the other end of the line was a fearful, stricken voice. Come quickly, the local told him, something very grave had happened. A Qur’an had been found in the shrine they had recently erected for the upcoming Hindu festival of Durga Puja. The Islamic holy book had been placed on a statue of the Hindu god Hanuman.

Das, a Hindu who organised the festival in Cumilla, felt dread rise up in him at the news of the desecration of Muslim holy scripture in their shrine. “It didn’t even take me a second to understand the gravity of the situation. I rushed there immediately,” he said.

The incident on the morning of 13 October set off some of the worst anti-Hindu attacks in years and left seven people dead. The violence seeped over the border into the neighbouring Indian state of Tripura, where more than a dozen retaliatory rallies by rightwing Hindu group Vishva Hindu Parishad (VHP) and others escalated into violence and anti-Muslim attacks. Muslim residents were terrorised, Muslim shops were torched, and at least 16 mosques were vandalised – of which four were set alight – in violence that began on 21 October and continued into last week.

VHP spokesman Vinod Bansal claimed reports of violence were “fake news” spread by “jihadists” and said it had only arranged peaceful rallies. But witnesses saw upwards of 3,000 people, many carrying sticks, iron rods, swords and cans of kerosene or petrol, marching through districts across Tripura last week, attacking Muslim homes and businesses. Saffron flags, the symbols of Hindu nationalism, were planted on several mosques, and pork – which is forbidden in Islam – was placed outside another.

The subcontinent has a long history of communal tensions and violence, from the bloody partition of India and Pakistan in 1947 to the violent 1971 war which led to the formation of Bangladesh. However, regional analysts say the latest incidents are indicative of the rising tide of religious intolerance against minorities in south Asia, evident across India, Bangladesh, Pakistan, Sri Lanka and Afghanistan.

In India, under the rule of the Bharatiya Janata party (BJP), led by prime minister Narendra Modi, the Muslim minority has found itself marginalised, attacked by politicians and subjected to discriminatory legislation as the BJP pursues a Hindu nationalist agenda.

In Muslim-majority Bangladesh, although the government of prime minister Sheikh Hasina is avowedly secular, it has been accused of playing a double-handed game with Islamist groups; cracking down on Islamist hardliners – and in particular the Islamic opposition parties – while also handling political Islam with caution, in order to appease powerful groups.

Hindus make up about 9% of the population in Bangladesh, and human rights groups have accused Hasina’s government of a creating a “culture of impunity” around attacks on the Hindu minority, over its repeated failure to hold anyone to account for incidents such as when hundreds of Hindu homes and temples were set on fire in 2016. “This region has become such a powder keg in terms of heightened risk of social instability and communal violence,” said Michael Kugelman, deputy director of the Asia programme at the Wilson Centre thinktank.

Kugelman directed blame at social media, which he said had become a “powerful proliferator of hate and toxicity” in south Asia, and was exacerbating volatile religious divides into violence.

Evidence suggests the initial incident in Cumilla was a deliberate sectarian provocation. CCTV appeared to identify Ikbal Hossain, a 35-year-old Muslim, as the one who placed the Qur’an in the shrine in the early hours, and he has now been arrested. But it was the streaming of the Qur’an incident on Facebook live, which was then shared and downloaded hundreds of thousands of times, accompanied by inflammatory calls for violence, which appeared to provoke the widespread riots.

Das described how an angry, agitated crowd of thousands began to gather outside the shrine. “I don’t know where all the people came from,” he said. “They started showering us with stones and bricks. Within minutes, they vandalised the idols. It was a complete wreck.” To Das’s horror, police officers then seized the remains of the idols and threw them into the nearby lake.

About eight temples and shrines were attacked that day, with molotov cocktails thrown through the windows and Hindus trapped inside for hours without help. The police, struggling to control the rioters, killed five people when they fired into the crowds.

Once the tensions had been ignited, it took five days for police to bring them under control as they spread across the country. Two days later, on 15 October, a mob of hundreds of thousands descended on Noakhali district and began attacking temples, beating up worshippers inside. In the famous Sree Sree Radha-Madhav Temple, the holy idols were disfigured and then trampled on, documents burned and cash boxes looted in attacks that lasted over two hours.

Ratan Lal Shaha, who is on the temple committee, was among those who accused police of failing to come to the aid of the Hindus. “Where were police when the attacks took place for hours?,” he said. “We called them, begged them to come.”

Like Das, he believed the attacks were organised. “Seven temples and four makeshift puja venues came under attack, more or less at the same time. The pattern of the attacks tells me it was very well-planned and well-coordinated.”

Two Hindus were killed that day, including Jatan Shaha, who was allegedly beaten to death at Sri Sri Radha Krishna Gour-Nityananda temple. His sister Mukta Rani, 41, described the attacks as “worse than a nightmare”. “It felt like the end of the world. They tried to force open our doors. They torched the temples,” she said.

Her brother, she said, had been lured outside by the attackers and then beaten to death. No police or ambulance would come to help. “I can’t believe it happened. What was the fault of my brother? He didn’t do any harm to anyone,” she sobbed. “Shaha’s son keeps asking when will his father return. It breaks my heart.”

Also beaten to death in the violence was Pranta Chandra Das, a devotee in an Iskcon temple complex, whose body was later found in a pond. His brother Shanto Das said: “I cannot put it in words how painful it feels. My little brother was a kind guy. He didn’t deserve this.”

Both families, along with Hindu community leaders, said they felt that Hindus were increasingly unsafe in Bangladesh. Many spoke fearfully of the growing number of hardline Islamist preachers on Facebook and YouTube who had vast online followings across the country.

Hasina has pledged to “hunt down” the attackers. Islami Andolan Bangladesh, an Islamist political party, condemned the attacks as carried out by those with “vested interests”, and said the Islamic community was not to blame. “These attacks weren’t communal, they were politically motivated. It was carefully done to destabilise the communal harmony of the Bangladeshi people,” it said.

Across the border in India, however, the response to the retaliatory attacks in Tripura was markedly different. Police, accused of enabling the violent VHP rallies, denied any riots had taken place. Those whose houses and businesses were attacked say the police are refusing to let them file cases against the rightwing Hindu groups responsible.

Nurul Islam Mazarbhuiya, Tripura chief of Jamat-e-Islami Hind, India’s largest Islamic organisation, said religious tensions were still running high, with another mosque targeted on Friday night. “There is every possibility of the outbreak of fresh violence,” he said.

Additional reporting by Shaikh Azizur Rahman