Syria's government has blamed "armed criminal gangs and terrorists" for much of the unrest since March 2011, but activists accuse state-sponsored militia of assisting in the crackdown that has left more than 9,000 people dead. Here is what is known about these armed regime supporters, known locally as the "shabiha".
Throughout the uprising, many Syrians have said they have seen heavily-armed men dressed in black fighting alongside the security forces.
The men are accused not only of killing and beating people who attend demonstrations, but also of carrying out a campaign of intimidation that has included executions, drive-by shootings and sectarian attacks.
Activists say their presence has allowed the government to deny any involvement in the most brutal actions against protesters.
"They're not afraid to use force, violence, weapons, racketeering and blackmail," Ammar Qurabi of Syria's National Organization for Human Rights told the Associated Press.
"That way, the regime will remain clean and will say: 'Look these are gangs doing this, not us'."
It is not clear exactly who they are and to whom they are loyal, but the term "shabiha" has repeatedly been used to describe them. Possibly derived from the Arabic word for "ghost" ("shabh"), it has come to mean "thugs" in modern day Syria.
The term is believed to have first appeared in relation to the uprising against President Bashar al-Assad after a crackdown was launched in the port city of Latakia, where a notorious, mafia-like organised crime syndicate called the Shabiha has existed since the 1970s.
In towns along the Mediterranean coast, local shabiha gangs are said to run protection rackets, weapons- and drug-smuggling rings, and other criminal enterprises. Residents reportedly dare not mention the name.
Membership of the shabiha gangs is drawn largely from President Assad's minority Alawite sect, which dominates the government, security services and military. Many are members of the Assad family itself, and the related Deeb and Makhlouf families.
In May, the European Union imposed sanctions on Mr Assad's first cousins, Fawwaz and Munzir, for their alleged involvement in "the repression against the civilian population as members of the shabiha".
The connection to the Assad family appears to have allowed the shabiha to emerge in the 1970s after Hafez al-Assad - father of Bashar - became president in a coup. The gangs operated with few constraints and were generally seen as being above the law and ruling by force.
Its members were reportedly armed by the Defence Brigades, paramilitary units commanded by Hafez's brother, Rifaat, who once owned a private port in Latakia.
However, by the 1990s the shabiha's disregard for the authorities in Latakia, and the brutal enforcement of their protection racket, reportedly became too much of an embarrassment for Hafez al-Assad.
Bashar was tasked by his father with reining in the shabiha, and restoring law and order to the city. In one incident, the future president allegedly had his cousin's bodyguards imprisoned for beating up a passerby.
Such actions were said to have curbed the worst of the shabiha's excesses, but its power and influence were not significantly eroded.
In late March 2011, the shabiha gangs appear to have come to the aid of President Assad when major anti-government protests erupted in Latakia, as well as the nearby towns of Baniyas and Jabla.
Residents said members had joined soldiers from the army's fourth armoured division - under the command of the president's younger brother Maher - and attacked civilians.
Gunmen fired automatic weapons from vehicles and took up sniper positions on rooftops, they added. Others broke into the houses of protesters, beat up occupants and set fire to the homes.
As the repression of dissent intensified, the meaning of the word "shabiha" expanded, with people from across Syria beginning to use it to refer to pro-regime militiamen who acted with impunity.
In May 2011, people who fled an assault on the western village of Tell Kalakh , which lies close to the Lebanese border, told reporters that some residents had had their throat cuts in the street by black-uniformed "shabiha".
Some of the attackers had been from Qardaha, a predominantly Alawite town in the north-west that is the ancestral home of the Assad family, and had been checking residents' ID cards in order to find local Sunnis, they said. Tell Kalakh is a main Sunni village surrounded by 12 Alawite villages.
"If they see he's a Sunni from his family name, they take him away and kill him," one woman told Reuters news agency.
"They destroyed the Omar Bin Khattab mosque because it is named after a companion of the Prophet Muhammad and dear to Sunnis. What we have here is a sectarian war between the Alawites and Sunnis."
Such claims raised suspicions among the opposition that the government was using shabiha to help it play up fears of a sectarian divide.
There were even reports that soldiers and police who tried to stop shabiha killing civilians in Tell Kalakh and elsewhere had been shot dead.
When an army brigadier-general was killed along with his two sons and a nephew in Homs in April 2011, officials blamed criminal gangs. But witnesses and human rights activists said he had been murdered by shabiha.
By June 2011, activists and witnesses said hundreds - even thousands - of shabiha had been sent to help the security forces launch attacks to crush dissent in restive cities, or intimidate those protesting elsewhere.
Videos posted online appeared to show men in civilian clothes join soldiers and police to beat and detain protesters in Homs . Others stabbed protesters gathered at a mosque in Damascus . Photographs were also published of burnt-out cars, homes and businesses, which were said to belong to known activists and protesters.
In May 2012, residents of Taldou, near the town of Houla in Homs province, said shabiha had been sent into their village after the Syrian army unleashed a barrage of heavy weapons in response to a local anti-government demonstration.
The UN Office of the High Commissioner for Human Rights said most of the 108 victims, including 49 children and 34 women, had been shot in their homes . No more than 20 had been killed by the tank and artillery fire which preceded the raid, it added.
"What is very clear is that this was an absolutely abominable event that took place in Houla, and at least a substantial part of it was summary executions of civilians - women and children," spokesman Rupert Colville told reporters .
Some witnesses said the shabiha came from the surrounding area, but this required further investigation, Mr Colville added.
Syrian officials have denied using pro-regime militiamen to intensify the crackdown on protesters and commit atrocities on its behalf.
They has instead insisted that criminal gangs or terrorists have targeted security forces, murdered civilians and destroyed property.
However, the claims have been dismissed by the international community, which has accused President Assad of "outsourcing repression".