They surrounded the building and forced an end to the meeting, before members were escorted away by police.

Ninety per cent of Indonesia’s 255 million people are Muslim but the nation is home to substantial religious minorities and several faiths are officially recognised.

It is these traditions that the NU, which has existed for almost a century, tries to defend.

It has been taking a more muscular approach by increasingly sending out its paramilitary wing Banser to take on the hardliners.

“My forefathers the clerics, as well as Christians and others, established this republic together,” said Banser’s national commander Alfa Isnaeni.

“We all need to defend this legacy.”

The NU says it has felt compelled to step in and expand its activities in part due to the weakness of the government, which has long been accused of failing to crack down on ultraconservatives.

There has been a growing number of attacks on minorities in Indonesia, from Muslim Shiites and Ahmadis to Christians, and concerns about intolerance surged after Jakarta’s Christian governor was jailed for two years for blasphemy, in a case seen as politically motivated.

Indonesia is not governed by Islamic law, with the exception of western Aceh province, and efforts by hardliners to transform the archipelago into a sharia-ruled state have gained no traction.

There is little chance of this changing – a recent survey showed only one in 10 Indonesians support a caliphate – but the surge in intolerance has caused jitters.

Members of Banser, which has a force about 2 million, do not carry arms but rely on sheer force of numbers to get their message across.

They confiscate banners and flags at rallies by hardline groups and hand them over to the police, justifying their actions by saying they are preventing conservative forces from trampling the country’s inclusive ideology.

They also oppose Wahhabism, an ultraconservative form of Islam that originated in Saudi Arabia and is practised to some extent by Islamic State. NU members have forced preachers who follow the doctrine off stage at public gatherings.

Their battle cry is “N - K - R - I” – the Indonesian acronym for the term “the United State of the Indonesian Republic”, highlighting their desire to keep the country together and strong.

“Anyone disagreeing with NKRI, or calling for a caliphate, will have to face us,” Isnaeni said.

In recent weeks, they have also helped protect several people targeted by hardline Muslim groups after posting anti-radical messages on social media.

The group holds rallies across Indonesia and has signed up thousands of new recruits to strengthen their efforts.

The organisation is not just fighting radicalism in the street but also on a theological level.

NU youth wing Ansor wants to open dialogue with Islamic organisations and governments to build a global consensus among Muslims on adapting the interpretation of ancient Islamic laws known as “fiqh” so that they suit the modern world.

It wants recognition among Muslims that followers of Islam and others are equal, and a focus on the importance of the modern nation state and a constitution as guiding principles for a country, as opposed to sharia law.

The NU’s efforts have sparked anger among conservatives, with some accusing them of being un-Islamic and defenders of non-Muslim “infidels” and Shiites, a Muslim minority regarded as a deviant sect by Indonesia’s mostly Sunni Muslim population.

NU’s secretary general Yahya Cholil Staquf believes promoting a more moderate form of Islam is urgent to tackle hardliners.

“We must fight them before they cause more damage,” he said. “We will fight this to the end.”