Hizb ut-Tahrir managed to fill the Gelora Bung Karno stadium

The dull roars of a football match, the twanging music of a youth group concert - from a distance it is not always easy to tell an Islamic conference from a holiday crowd.

Inside Jakarta's Gelora Bung Karno stadium the clues get easier. There are about 100,000 people inside, and everyone is in Islamic dress.

The women's section - by far the largest - is a pitter-patter of ice-cream colours. On their parasols, one word is printed over and over again: Khilafah, caliphate.

This is the reason why people have come here. To show their support for a single, unified, Islamic state.

Maybe I chose Hizb ut-Tahrir because it unites the masses better than other Islamic organisation Yani, Hizb ut-Tahrir member Q&A: Hizb ut-Tahrir

They have been invited by the international Islamist group, Hizb ut-Tahrir. Not everyone believed they would fill the stadium, but Hizb ut-Tahrir is good at bringing in supporters - and keeping them.

Milling around outside the stadium we found 24-year-old Akbar.

He was not a member of Hizb ut-Tahrir, but he said: "This conference is not just for one group. In my opinion, if you support there being sharia law in Indonesia, you've got to be here."

Yani, a student from Bogor, said she had come to show there was support for Islam, and support for a Caliphate too.

It can take several years to gain membership of Hizb ut-Tahrir

Next to her, Wisnu told us she was there to increase ties with other Muslims.

"Maybe I chose Hizb ut-Tahrir because it unites the masses better than other Islamic organisations," she said. 

But if the audience turnout was impressive, not so the speakers lined up to address the crowd.

One by one, over the past few days, seven of the delegates invited to speak have dropped out.

'Uncommon democracy'

One of those who did turn up to speak was Din Syamsuddin - an establishment figure rather than a firebrand, and leader of Indonesia's second largest Muslim organisation, Muhammadiyah.

Many Hizb ut-Tahrir member are from Indonesia's middle class

But this was a conference that would like to overturn Indonesia's democratically elected government and install an Islamic state - so where does he stand on that?

"I think democracy is OK," he said. "But it's not enough. I think democracy in Indonesia should be supported by religious, ethical and moral values."

"Because this is a country where the majority of its citizens are religious people. So maybe not liberal democracy, but uncommon democracy; based on religious values - I say religious values, not necessarily Islamic values."

There was a lot of speculation before this conference began about what kinds of messages would be reflected here.

Hizb ut-Tahrir says it is not an extremist organisation: it does not have a paramilitary wing, and has never been charged with violence.

But its hardline agenda and rhetoric, and its secretive recruitment process, have won it many opponents.

Educated classes

Kholid has been a member of Hizb ut-Tahrir in Indonesia for six years. He joined at college and says the teachings of the party have changed the way he views the West.

"It comes as a matter of course," he told me. "I've come to believe that Muslims have the right to defend themselves when attacked, but we're not allowed to be aggressive against Westerners if they're not attacking us. 

"The method used in Hizb ut-Tahrir is a change in thought patterns. We call it 'thought revolution'. When someone is given Islamic teaching - given the brilliant thinking of Islam - then they'll naturally undergo a thought revolution, and will see what is good and what is bad."

Hizb ut-Tahrir will not say how many members it has. But those close to the group say membership is extremely difficult to win - often taking several years.

Unlike many other Islamist movements here, Hizb ut-Tahrir seems less interested in a broad mass following than a smaller more committed core of members, many of them drawn from Indonesia's educated middle classes.

The organisation has only been operating openly in Indonesia for seven years. The conference is one sign that it is doing well.