JAKARTA: A defiant declaration of atheism by an Indonesian civil servant has inflamed passions in the world's most populous Muslim nation, pitting non-believers and believers against each other.

The trouble began when civil servant Alexander Aan posted a message on the Facebook page of Atheist Minang, a group of Indonesians with godless beliefs.

The post so enraged residents in Aan's hometown of Pulau Punjung in West Sumatra province that an angry mob of dozens stormed his office and beat up the 30-year-old.

To add insult to injury police then arrested him and now want to press blasphemy charges that could see him locked up for five years.

Religious extremists have called for Aan to be beheaded but fellow atheists have rallied round, and urged him to stand by his convictions despite the pressure.

“Dear Alex, stick to your beliefs. This country has no right to restrict your faith,” Fahd Singa Diwirja wrote on the same Facebook page, where Aan is one of the administrators.

“You're facing narrow-minded people, but this is the true Indonesia, a fertile ground for the spread of fundamentalism,” Diwirja added, advising Aan to escape persecution by seeking asylum in a European country.

Aan has also gained the support of the US-based International Atheist Alliance.

The group, together with Atheist Minang, has written to President Susilo Bambang Yudhoyono, calling on him to ensure that the blasphemy allegations are dropped.

“This is a law that has been used to promote mob violence and intimidation against those who do not agree with...vigilante groups,” said the letter, copies of which were also sent to the United Nations and Human Rights Watch.

Aan's proclamation has been removed from the page, but the Facebook group has doubled to 2,000 since the controversy made local news reports.

Most of the postings, however, are diatribes against Aan and his ilk.

“These atheists should be beheaded, that's what they deserve,” wrote a man who identified himself as Putra Tama, a Muslim from neighbouring Jambi province.

Other posts challenged atheists from the group to dare show themselves, instead of hiding behind the anonymity of social media.

“If you think your arguments are true, why don't you just have a face-to-face meeting with us, people who still believe in God? You're just a group of cowards,” taunted another post.

Although Indonesia's constitution guarantees freedom of religion, it only recognises six faiths: Islam, Buddhism, Hinduism, Catholicism, Protestantism and Confucianism.

Perceived blasphemy against any one of these religions carries a maximum five-year jail term.

Local police chief Chairul Aziz said this week that Aan, who had written on his Facebook page that he was brought up as a Muslim, had expressed his willingness to revert to Islam but that it would not be enough to escape punishment.

“He expressed his intention to convert to Islam but he has not performed an Islamic declaration of faith. Even if he does so, he still can't escape from justice due to his blasphemous act,” Aziz told AFP.

He said Aan could face additional charges, including falsely declaring himself a Muslim when he applied for a civil service job years ago.

The Islamic Society Forum (FUI), an umbrella group for several hardline groups, said that a five-year jail term for Aan would not suffice.

“He deserves the death penalty, even if he decides to repent. What he has done cannot be tolerated,” said Mohammad al-Khaththath, FUI's secretary-general.

“It is important to prevent this group from spreading atheism in this country,” he added.

Indonesia has seen a spate of attacks on minority religious groups in recent years and the country's judiciary is notoriously unsympathetic towards their plight.

In 2011 three Ahmadiyah sect members were killed in a mob attack.

Those convicted received light sentences of between three and six months while one of the Ahmadiyah survivors, a man who almost lost his hand in the violence, got six months for defending himself and his friends.