Meet Invincible Voice (I-Voice), two Palestinian rappers with an international following and more than 20,000 hits on their MySpace page.

Yasin Qasem, a 21-year old freelance sound engineer, and TNT aka Mohammed Turck, a 20-year-old foreman, have little difficulty touring Europe. But when the duo recently wanted to perform in Morocco they were denied entry. Yasin could not even get a visa to lead a sound engineering workshop in Casablanca. Earlier this year, the duo obtained visas for Dubai to produce their upcoming album but were turned back at Dubai airport.

Yasin and TNT have two strikes against them. They are Palestinians and they perform music that is viewed by authoritarian governments as subversive.

From Morocco to China, heavy metal musicians and their fans have been arrested and accused of threatening public order, undermining Islam and performing the devil's music. Last summer, police in Riyadh broke up a heavy metal concert in a residential compound attended by 500 mostly Saudi fans.

The highly-charged music nonetheless lives on in underground clubs, basements and private homes. 'As musicians push the boundaries of acceptable musical performance in their countries, it is clear that, wittingly or not, they are helping to open their cultures and potentially their political systems, along with them,' says Marie Korpe, executive director of Freemuse, an organisation promoting freedom of musical expression.

In a world with a dearth of outlets to express discontent, heavy metal offers an opportunity to resist authoritarian political and cultural regimes in which fans feel estranged or marginalised.

'We play heavy metal cause our lives are heavy meta'l, says Reda Zine, one of the founders of the Moroccan heavy metal scene. A Chinese colleague adds: 'Youngsters can express their hatred and emotions through metal. The music of Chinese metal groups reflects injustice, political inadequacy and corruption in government.'

The two musicians are quoted by Mark LeVine, who is a University of California Middle East history professor, an accomplished musician who performed with the likes of Mick Jagger and Albert Collins, and author of a just released report entitled Headbanging Against Repressive Regimes: Censorship of Heavy Metal in the Middle East, North Africa, Southeast Asia and China.

Boosted by the internet and technologies that facilitate mass distribution and are difficult for governments to control, underground music in the Middle East and North Africa prompts reminders of the role music played in the velvet revolution that toppled regimes in Eastern Europe and the Suharto regime in Indonesia. It 'reminds us of a past, and offers a model for the future, in which artists — if inadvertently at first — helped topple a seemingly impregnable system of rule', LeVine says

Underground music, he says 'are avatars of change or struggles for greater social and political openness. They point out cracks in the facade of conformity that is crucial to keeping authoritarian or hierarchical and inegalitarian political systems in power.'

The music enables musicians and their fans to carve out autonomous spaces resistant to intrusion by government and conservative culture. To some, its rituals and practices demonstrate affinity with those of Islam. 'I don't like heavy metal. Not because it's irreligious or against Islam; but because I prefer other styles of music. But you know what? When we get together and pray loudly, with the drums beating fiercely, chanting and pumping our arms in their air, we're doing heavy metal too', a Baghdad Shi'ite cleric tells LeVine.

That association makes heavy metal even more subversive. In a crackdown that until today puts its stamp on the heavy metal scene in the Middle East and North Africa, police in Cairo in 1997 arrested 100 heavy metal fans. The arrests followed publication of a photo from a metal concert allegedly showing someone carrying an upside down cross. Egypt's then mufti, Sheikh Nasr Farid, demanded that those arrested repent, or face the death penalty for apostasy. Intimidated, musicians and fans destroyed their guitars and shaved off their beards to avoid arrest or worse.

A decade later, many musicians remain reluctant to publicly discuss their music or lyrics even though government policy is somewhat more relaxed because President Hosni Mubarak is more concerned about the Muslim Brotherhood and bloggers than it is about underground music.

Unlike in Egypt, Morocco's heavy metal scene successfully resisted government repression. In 2003, when authorities sentenced 14 musicians on charges of being Satanists, the scene responded with mass protests. The government was forced to overturn the verdicts in a rare civil society victory against an Arab government.