The phone calls began two months ago from an unknown number. Hilmy Ahmed, the vice-president of the Sri Lanka Muslim Council, did not recognise the voice but there was no mistaking the threats: throw your support behind Gotabaya Rajapaksa for president or we will set fire to your house, rape your wife and then kill your family.

“This election campaign has the whole Muslim community of Sri Lanka very scared,” said Ahmed, who fled abroad after the calls and direct harassment of his staff. “Almost all the nationalist extremists have banded together behind Gotabaya Rajapaksa and should he win, we fear they will come out in full force with their racist, anti-Muslim agenda and attacks.”

Security personnel escort Sri Lankan polling officers carrying election materials as they leave for their respective polling booths on the eve of the presidential elections in Colombo, Sri Lanka, 15 November 2019. Photograph: STR/EPA

On Saturday, voters in Sri Lanka’s general election will choose between Gotabaya Rajapaksa, the brother of the former president Mahinda Rajapaksa, running for theSLPP, the Sinhalese-Buddhist nationalist party, and Sajith Premadasa, a minister in the current United National party (UNP) government, to be their next president.

The election, happening against a backdrop of some of the worst violence and political instability the country has faced since the end of the civil war a decade ago, could prove a decisive moment, with everything from human rights to sectarian harmony hanging in the balance. Most precarious of all is the place of Muslims in Sri Lankan society.

On Easter Sunday this year, self-radicalised Islamist extremists carried out multiple bombings, killing 269 people in churches and hotels and shaking the foundations of the country, pitting communities who had lived harmoniously against each other.

Sri Lankan soldiers inside St Sebastian’s Church at Katuwapitiya in Negombo on 21 April 2019, after a bomb blast during the Easter service. Photograph: AFP/Getty Images

Since then, the lives of many Muslims have been thrown into turmoil. Some were attacked and shops and homes were destroyed by Sinhala Buddhist mobs, and some were arbitrarily arrested. Women were banned from wearing headscarves in public, and in July newspaper front pages carried the unsubstantiated story of a Muslim doctor who had allegedly sterilised 8,000 Sinhala Buddhist women without their knowledge.

For many people who lost family in the Easter attacks, reconciliation with their Muslim neighbours seems impossible. Sitting in a starkly lit fast-food restaurant in Colombo, Gloriya George, 19, clutched her hands tightly as she recalled sitting in the pews of St Anthony’s shrine as a bomb went off. Her father, Narayanan George Chadrasekaran, was among those killed.

“After this attack which killed my dada, everyone where I live stopped talking to the Muslims because we are very scared,” said George. “Among my friends who are Christians, we won’t go to Muslims’ restaurants and we won’t even get in the car with an Uber driver if he is Muslim now. It is sad because we always got along before. My dada only worked with Muslims, they were his friends, and they would bring biriani to our house during Ramadan. But we do not speak to them any more.”

Both Rajapaksa and Premasada have played heavily on their Sinhala Buddhist credentials, giving the election overtly religious nationalist overtones, but it is Rajapaksa’s strong security agenda that has Muslims – who make up 9% of the population – most worried. The race is tight but Rajapaksa is the favourite to win.

Gotabaya is one of four Rajapaksa brothers who have dominated Sri Lankan politics for more than a decade. He held the position of defence secretary when his brother Mahinda was president from 2005 to 2015, and the pair were credited with ending the civil war between separatists from the mostly Hindu Tamil minority and the Sinhala Buddhist-dominated government, which had lasted 26 years and cost at least 100,000 lives.

However, Mahinda’s presidency was also marked by widespread human rights violations, oppression of dissent and attacks against journalists and campaigners. Some of the brothers, including Gotabaya, are still facing charges of corruption and fraud.

Mahinda remains the more popular brother but Sri Lankan law prevents him from running again. Gotabaya, who is notorious for being the most militaristic and nationalistic of the family, was put forward instead.

The Rajapaksa campaign confirmed to the Guardian it was hoping to put forward Mahinda as its candidate for prime minister in elections next year, paving the way for a double Rajapaksa hold on power.

In the small town of Minuwangoda, just north of Colombo, Rinzan Mohideen, 38, described how his shops were among 20 Muslim-owned businesses burned to the ground during anti-Muslim riots in May, costing him 19m rupees (£82,000) in damages. “If Gotabaya wins then I will pack up and leave because things will only get worse and worse for us, there will be no hope for the Muslim community. I cannot let my daughter grow up in that environment,” he said.

The Rajapaksas denied they had a Buddhist nationalist or anti-Muslim agenda. “The Muslims will have nothing worse than their experience under this current government,” Basil Rajapaksa, one of the brothers and the chief strategist of Gotabaya’s campaign, told the Guardian in an interview. “But unfortunately most Muslims are not realising this and their leaders mislead them. Their leaders are only worried about their own position because they can’t auction the Muslim vote to us to get something in return.”

The hostility towards Muslims has been simmering for several years in Sri Lanka, stoked primarily by Buddhist nationalist groups such as Bodu Bala Sena (BBS). Firebrand monks have propagated the idea that Sinhala Buddhists are under threat from overpopulating Muslims who plan to control the economy, overthrow them and then wipe them out.

SLPP supporters cheer during Gotabaya Rajapaksa’s election rally in Jaffna. Photograph: MA Pushpa Kumara/EPA

The ideology remained out of the mainstream, but since the Easter attacks such religious nationalist ideas have gained traction in Sinhala communities. These groups, who initially thrived under Mahinda Rajapaksa’s presidency, have now thrown their support behind Gotabaya.

As a result, if the Rajapaksas return to power, many Muslims in Sri Lanka fear a similar fate to the minority Muslim Rohingya community in Myanmar, where Buddhist nationalist monks – who follow the same purist Theravada strain of the faith – were responsible for stirring up much of the racist violence that led to ethnic cleansing and tens of thousands of Muslims killed in 2017.

Shuhaib Ali, 31, a Muslim teacher from Colombo whose aunt was killed by the Easter bombers while at breakfast in the Shangri-La hotel, said: “Anti-Muslim hate has spread here like a virus and all the nationalist talk in this election has made it even worse.”

Ahmed said: “There might be nothing to stop a Myanmar-style genocide.”