Many of the paramilitary units, which have been firing indiscriminately at civilian protesters, have long been part of Libya's internal security system and have helped as a check on any uprising by the army. Now they are bloodying civilians.
"You have the traditional army and you have a parallel army," said Noman Benotman, a senior analyst at Quilliam, a London counter-extremism think tank, and a former Libyan jihadist. "The army is weak, not a significant power. The parallel units are controlled by the most loyal people, not just to the regime, but to Colonel Gaddafi personally."
Unlike the Egyptian army, which is a strong and independent institution, the Libyan army has been subject to overweening political controls.
Resources have flowed to loyalist units while other sections of the military have been effectively mothballed.
In the ongoing crisis, some of the loyalist units have been turning to foreign fighters for assistance.
The Khamis Brigade, named after Gaddafi's youngest son, was reported to be flying in additional mercenaries from African countries as recently as Wednesday, according to Omar Khattaly, a co-founder of the Libyan Working Group, an exiled human rights group with offices in Atlanta and Europe. Some of the mercenaries were landing at what used to be the U.S. Wheelus Air Base, near Tripoli.
Identity cards from Guinea, Niger, Chad, Mauritania and Sudan were reportedly found on individuals wearing Libyan uniforms and killed in the eastern city of Benghazi and other locations.
"I think the history of foreign mercenaries is one of the ability of those individuals to detach themselves from local conditions and to be relatively ruthless," said Juan Zarate, a deputy national security adviser in the administration of George W. Bush. "And in Libya, it's a thug corps."
The exact number of paramilitary forces at Gaddafi's disposal is unknown. According to one estimate, there is a 3,000-man Revolutionary Guard Corps plus unknown numbers of fighters in the Islamic Pan African Legion, the People's Cavalry Force and various "people's militias."
"Libya is better at internal repression than in dealing with foreign threats," Anthony H. Cordesman and Aram Nerguizian of the Center for Strategic and International Studies in Washington wrote in a study called "The North African Military Balance," in which they included the estimate. They noted that paramilitary forces "act as a means of controlling the power of the regular military and providing Gaddafi with security."
The Pan African Legion, which Gaddafi established decades ago, has historically drawn fighters from across Africa and participated in a number of small wars when the Libyan leader attempted to project some regional power during the Cold War.
After the United States reestablished full diplomatic relations with Libya in 2006, the Gaddafi family began to press American officials for the resumption of military sales. One of Gaddafi's sons, national security adviser Mutassim Gaddafi, urged U.S. officials to sell Little Bird attack helicopters to the Khamis Brigade.
Gaddafi suggested that all armaments be removed from the helicopters so they could be categorized as "non-lethal equipment," to facilitate the transfer, according to a diplomatic memo released by WikiLeaks. The United States refused to sell.
Foreigners have also played a role in security forces elsewhere in the region, including in Bahrain, another country facing a popular revolt, one tinged with sectarian divisions. The kingdom's security forces have long been staffed with Sunni Muslims from Pakistan, who often harbor a deep religious hostility to Shiites, whose anger has helped fuel demonstrations in Bahrain.
The Bahrain Center for Human Rights, a nongovernmental organization, has complained about the kingdom's recruitment drives in the Pakistani province of Baluchistan for the island's national guard and special forces, saying that these "foreign mercenaries" are "used to suppress local movements petitioning for greater rights."
Under international pressure, violence by the security forces in Bahrain has abated.
Army in bad state
In contrast, pressure from abroad carries little weight in Libya, and the army has been unable to act as an internal check on the police and security services.
"Libya has to keep many of its aircraft and over 1,000 of its tanks in storage," according to Cordesman and Nerguizian. "Even its best combat units are under strength and have severe training and leadership problems."
The poor state of the regular army led the Quilliam think tank to conclude in a report this week that "reports of army units joining the demonstrators are therefore in themselves not necessarily significant or fatal to the regime on their own."
Staff writers Craig Whitlock and Jeff Stein and staff researcher Julie Tate contributed to this report.