"The Shabiha are a tool which allows the regime to behave in the most uncivilised way without pinning it to an institution," said a Damascus-based analyst, who requested his name be concealed for fear of reprisal.

"They'll beat up the elderly, children, and then the regime can say, 'it's not us'," the expert told AFP.

"The regime's extreme thuggishness is cloaked behind a thin veil of deniability."

While the Shabiha hold no official affiliation to the ruling Assad dynasty, the regime has outsourced the dirtiest of its work to the groups, who have been wreaking havoc in towns across the country, armed with everything from clubs to automatic weapons.

Syrian refugees in Lebanon have brought with them tales of horror at the hands of the feared troops of Maher Al-Assad, younger brother of President Bashar al-Assad, and, increasingly, at the hands of the so-called Shabiha.

Several refugees interviewed by AFP have given reports of what they say are arbitrary detentions and executions by the Shabiha, describing a state of spiralling lawlessness back home.

Videos posted on YouTube, which has emerged as a major source of news on Syria due to a state-imposed media blackout, often show men in civilian clothing brutalising protesters outside mosques in cities from Homs in the centre of the country to Daraa in the south.

It is often difficult to discern plainclothes security forces from civilian supporters of the regime, whose troops have cracked down on mainly Sunni Muslim protesters demanding an end to nearly half a century of rule by the Alawite-controlled Baath Party.

A further complication is the fact no one can say for sure who the Shabiha are or how they are organised.

"The Shabiha are not one, unified group, like a troop," said activist Omar Edelbi, spokesman for the Syrian Local Coordination Committees, an anti-regime activist network, and member of the opposition Syrian National Council formed in Istanbul last week.

"These are armed civilian gangs recruited from different cities and sects across Syria that are fighting and killing their fellow Syrian civilians for largely ideological reasons - gangs that, unlike the army, are accountable to no one," Edelbi told AFP.

"We Syrians can tell the difference straight away. But for outsiders, it is almost impossible."

The origin of the word Shabiha points to the Arabic root "shabah," or ghost, and first surfaced in Syria's Latakia decades ago to describe gangs of smugglers who ran amok in the then-scenic coastal city.

"The gangs we are seeing in the streets of Syria today are a different phenomenon from the mafias of Latakia in the 1980s, which the Assads in fact suppressed," Edelbi noted.

"The Mercedes Benzes the Latakia gangs used to drive were dubbed 'shabah' because they were so fast, and so the gangs themselves came to be called 'shabiha', which is now used to describe a different phenomenon altogether."

Today, experts say, one of the Syrian regime's last cards are these teams of civilian defenders, who hail from all sects and economic classes.

"The regime was quite dexterous in the first months of this uprising," said Joshua Landis, director of the Centre for Middle East Studies at the University of Oklahoma and author of the blog Syria Comment.

"The mindset in the regime has long been that you can only trust an Alawite ... but these sorts of 'reserve soldiers' can come from anywhere," Landis told AFP.

"The Shabiha's role ... is basically intimidation and restoring the fear that had held Syria together for the past 40 years, which seems to have broken with the Arab Spring."