Accused of the most barbaric massacres since the start of the revolt in Syria, the shabiha are feared militiamen and tools of a regime seeking to dissociate itself from atrocities, experts and activists say.
While there is no hard evidence of the involvement of these gunmen in the repression, United Nations officials have expressed "strong suspicions" about their role, notably in the Houla massacre that left 108 dead on May 25 and 26.
The regime of President Bashar al-Assad has denied any connection with the carnage, which it blames on "armed terrorist groups."
"The shabiha are those who carry out the regime's dirty work. The government can say 'this is not me, I am not responsible'," said Fabrice Balanche, director of the French Research Centre Gremmo.
"They provide cover for the regime when massacres are committed," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.
The word "shabiha" alone is enough to make people tremble. These men, often in plain clothes, are also accused of arbitrary arrests, summary executions and torture.
"I do not think the Damascus regime actually says 'now is the time to commit massacres'," said one analyst in Damascus on condition of anonymity.
"But for 15 months, the authorities have not only tolerated, but exploit the phenomenon of the shabiha, contributing greatly to the deterioration of the crisis."
According to experts, this phenomenon is not unique to Syria, but has thrived in countries dominated by totalitarian regimes or experiencing unrest.
"It's like death squads in Latin America. It is also a way to terrorise people," said Balanche.
The word "shabiha" (from the Arabic "shabah," which literally means "ghost"), dates to the 1980s when it referred to drug traffickers who sped through the port of Latakia in Mercedes cars and who, activists say, worked for the Assad clan's personal circle.
They reappeared when the anti-regime revolt broke out in mid-March 2011, this time to quell protests.
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Members of the militia are mostly "young, unemployed men from the suburbs who, given money and a Kalashnikov, feel they are all-powerful," said Balanche.
According to Dany Hamwi, an activist in the central city of Hama, the regime uses the shabiha to spare the army from direct involvement in crimes and thus reduce the chance of mass desertions.
"An officer or soldier might refuse orders to kill, but a 'shabih' is loyal to the end," he said.
A number of activist videos show men in civilian clothes, armed with sticks or Kalashnikov assault rifles, attacking protesters and chanting: "Shabiha for ever, for your eyes, Bashar!"
According to the Syrian Observatory, the number of shabiha is estimated at nearly 6,000, some of whom have been integrated into the security services.
By fighting for the regime, they are also battling for their own survival, in part because they are predominantly Alawites -- the confession of the Assad clan -- facing Sunni opponents, the majority religious community in the country.
Activists have accused Alawite shabiha from neighbouring villages of committing last month's massacre in the small central Syrian Sunni farming community of Houla.
"These are the biggest supporters of the regime," said Balanche. "And the Alawites are terrified of Sunni vengeance" if the regime falls.
According to activists, a Mafia mentality now prevails.
"The regime used to pay them wages. But with the economic crisis, it has given them the green light to plunder neighbourhoods," said Omar Shakir, an activist in Homs.
Kidnappings and robberies have become commonplace in increasingly militant Aleppo, Syria's second city.
"The 'contract' was: 'You make sure there are no demonstrations, and in return, you do as you please'," said the Damascus-based analyst, adding that "this has been a large factor in turning Aleppo against the regime."
The risk, he said, is that the regime can no longer control "the Frankenstein that it alone has created."