BBC News Tehran
The lights are dimmed in a home in northern Tehran. The men, women and teenagers gathered in the large living room close their eyes and rock back and forth to the beat of live music.
As the tambourine and drums beat louder and faster, some members of the group climb to their feet. They begin to swirl slowly in circles and raise their hands to the ceiling. A few fall into trances.
These Iranian Sufis believe music and dance can provide a direct route to Allah
"You can somehow touch relaxation," says 22-year-old Mahsa, who believes that music and dance can provide a direct route to Allah.
"It's a very good sensation, and you think your soul is flying, that somehow you're not in your body."
These Iranians consider themselves Shia Muslims, as do most Iranians, and look to the first Shia Imam, Ali, as a spiritual guide.
But they also call themselves Sufis.
Sufis believe that at the core of all religions lies the same truth and that God is the only reality behind all forms of existence.
They also believe that the individual, through his or her own efforts, can reach spiritual union with God.
Spread of Sufism
Sufism, or Islamic mysticism, appeared in the eighth century in present-day Iraq.
Iranian Sufis say Islamic mysticism has become more and more popular in the country in recent years.
No official statistics are available, but Heshmatollah Riazi, a former professor of philosophy and theology in Iran, believes two to five million Iranians practice Sufism today - compared to only about 100,000 before Iran's Islamic Revolution of 1979.
He says Iran is home to the largest number of Sufis in the Middle East.
"Sufis have nothing to gain from superficial religious thoughts, and they seek spirituality," says Mr Riazi.
Nowadays, hundreds of young Iranians are increasingly joining Sufi groups.
"They need something to develop love and their internal sense of freedom."
Some Iranians who are attracted to Sufi sessions say their gatherings provide entertainment and camaraderie.
Others say they like Sufism for its liberal view of religion.
"Official religion has a series of limitations, and its limitations are much stricter than in Sufism," says 20-year-old Ashkan, a member of the New World Unity Sufi group in Iran.
Many Iranian Sufis also report that the growing popularity of Sufism has contributed to greater tensions between them and certain elements of the Islamic regime.
Earlier this year, violent clashes broke out after authorities ordered the closure of a Sufi house of worship, or Husseinieh, in the central Iranian seminary city of Qom.
Members of the Nematullahi Gonabadi Sufi group say the Husseinieh was set on fire and that hundreds of their members who had refused to leave the building were arrested, although most have been released since then.
Qom's governor, Abbas Mohtaj, has said the Sufis were ordered to leave because they had begun building on the property without the necessary permits, the government-run Iran newspaper reported on its website.
Mr Mohtaj has also been quoted as saying the Sufis had ties to foreign countries that were trying to create insecurity in Iran.
But Riazi denies these charges and says the authorities had other motives for shutting down the Husseinieh.
"[The authorities] are concerned that Sufis will do something against them, while on the contrary, Sufis don't interfere in politics at all," says Mr Riazi, a member of the Gonabadi group, which does not have music or dancing at its gatherings. "They follow the law and are not opposed to the Islamic Republic."
Sufism and orthodoxy
In the past, certain Sufi groups have come into conflict with orthodox Islam.
They caused concern among some clergy over the observance of practices that departed from traditional ritual.
Some Sufis say before Iran's Islamic Revolution, Mohammad Reza Shah imprisoned some Sufi leaders.
They say in the early years after the revolution, the new regime also confronted some Sufi groups - detaining some leaders and shutting down their gatherings.
Sufi musicians at a gathering in Tehran
Sufis look to their own spiritual leaders, while Iran's official version of Islam advocates the practice of following a Marja-e taqlid, or a cleric who is an expert in Islamic jurisprudence.
And in contrast to Sufis, orthodox Muslims believe that a person can never "become" God or be united with him.
Many Sufis say, however, the Qom dispute was an exception to the generally cordial relations they have had with the government in recent years.
And Javad Arianmanesh, a member of the Cultural Commission in Iran's parliament, says the government does not limit Sufi activities.
"Sufis are one of those groups which mostly act within the framework of Islam," he says.
"Also, because Sufis work within our framework [that of the Islamic Republic], there is no kind of problem for them."
"Iran - contrary to the propaganda that the world spreads against it - is one of the freest countries of the world, and Sufis also are part of this country and are completely free," he says. "They can, based on their own beliefs, perform their own ceremonies."
But a government official who preferred to remain unidentified says the regime's treatment of Sufis is not always consistent because it is based on ambiguous laws.
He explains that on the one hand, Sufi meetings should not be disrupted, but on the other hand, Sufis should not proselytize.
"We asked both the previous and the current governments to have more concrete laws and to form a commission to oversee Sufis' activities because regulations about their activities are contradictory," says the official, who worked for both the previous, reformist President Mohammad Khatami and the current, conservative President Mahmoud Ahmadinejad.