Libya: Who is propping up Gaddafi?
Col Muammar Gaddafi's regime is showing signs of fighting back.
His air force has been in action over the eastern town of Ajdabiya, his paramilitaries have been manoeuvring around the western towns of Misrata and Zawiya, and fighting has erupted around the oil terminal at Brega.
The opposition says military reserves have been moved up from Sebha and other parts of the south that are under regime control, and there are signs that Col Gaddafi has not yet deployed the full force at his disposal.
So what exactly can the Libyan leader call on, and how do those forces match up to what his opponents have?
Like other Arab strongmen rulers in recent history - Iraq's deceased President Saddam Hussein, Syria's deceased President Hafez Al-Assad, and Yemen's ruling President Ali Abdullah Saleh - Col Gaddafi places much of his trust in those of his own flesh and blood, empowering three of his sons with key military roles.
The Khamis Brigade
In this internal conflict, where Libyan is fighting Libyan, Col Gaddafi's power rests not so much on the mainstream army, which is weak and ill-equipped, but on his loyal paramilitaries.
The best trained and the best equipped unit (according to leaked US embassy reports) is the 32nd Brigade, known as the Khamis Brigade, a so-called Special Forces unit commanded by the Russian-trained Khamis, the fifth of Col Gaddafi's seven sons.
It has modern Russian-built tanks and Grad rocket-launchers, mounted on the back of trucks, capable of unleashing a fire storm of explosive on an area the size of a football pitch in a short space of time.
Its troops, numbering a few thousand, are better paid than others, and the unit has been described as a Praetorian Guard for the Gaddafi family.
So any large-scale desertions or defections from it would be significant.
Other paramilitary units include the People's Security Organisation and the Revolutionary Guards Corps which, according to IHS Jane's Country Risk analysis, numbers about 3,000 with access to a variety of weapons.
Like nearly all Arab countries, Libya has an extensive, well-resourced and brutal internal security apparatus.
Think East Germany's Stasi or Romania's Securitate pre-1989, where no-one dared criticise the regime in public in case they were reported to the feared secret police, and you can see the similarities.
During my own visits to Libya I have always found it hard to get ordinary people to speak freely on the record to a journalist, as government "minders" are always watching and noting who says what.
A key figure in Libya's security apparatus for years, both internal and external, has been Col Gaddafi's brother-in-law, Brigadier General Abdullah Senussi.
A hardliner with a thuggish reputation, he is strongly suspected of being the driving force behind the violent suppression of protests, notably in Benghazi and the east of the country.
On Tuesday there was an unconfirmed report that he had been sacked, which again would be significant if confirmed.
Another Gaddafi son, the Egyptian-trained Col Mutassim was made head of Libya's National Security Council in 2009. It was a rehabilitation for him, as he had fallen out with his father 10 years earlier.
The other sons
Saadi Gaddafi, a Colonel, was despatched to Benghazi and the rebellious east last month to try to reverse the losses sustained early on by the regime there. There has been little news of him since.
Another son, the wayward Hannibal whose bodyguards recently wrestled with Swiss police in a Geneva hotel, holds a low military rank and according to research by US-based think tank the Jamestown Foundation, plays only a minor role in the military.
This has been one of the darker and particularly disturbing facets of the Libyan uprising.
There are persistent reports that the Gaddafi regime has been making extensive use of hired African mercenaries, mostly from the Sahel countries of Chad and Niger, to carry out atrocities against unarmed civilian protesters.
Libyan witnesses say they have been firing from rooftops into crowds of demonstrators - in essence carrying out the orders that many Libyan soldiers have refused to obey.
Col Gaddafi has spent years fostering close relations with African leaders including Zimbabwe's Robert Mugabe, having long ago given up on his dreams of forging Arab unity.
Estimates of the number of African migrants in Libya are upwards of 500,000 out of a total population of six million.
The BBC Hausa language Service has interviewed nationals of Niger living in Libya who say that sub-Saharan Africans are being rounded up and forced to either join Col Gaddafi's army or be killed.
The exact number of those voluntarily serving as pro-Gaddafi mercenaries is unknown, but their motivation comes down to one thing - money.
There have been reports of extra flights being laid on to bring in more in recent days, but the Gaddafi regime has flatly denied using mercenaries. (Col Gaddafi has also denied there have been any demonstrations against him).
Most of Libya's armoury is composed of low-tech ageing Russian hardware, predominately AK-47 assault rifles and RPG7 anti-tank rockets.
This has allowed untrained but highly motivated opponents to overwhelm regime forces in the east by sheer force of numbers and determination.
In the fluid tug-of-war that defines this conflict, military movements have been characterised not by well-planned mass movements of armoured columns supported by air power, but by rapid dashes across the desert by lightly armed men in pick-up trucks and SUVs.
The bulk of more powerful weaponry is in regime hands, including several hundred main battle tanks such as T-72s, armoured personnel carriers, attack helicopters and Russian and French-built warplanes.
For now, the regime's airstrikes seem to be concentrated on trying to destroy ammunition depots that have fallen into opponents' hands.
Senior deserting officers have been warning that Col Gaddafi and his family will stop at nothing to stay in power, including dropping bombs on civilians.
This may well be a factor driving UK Prime Minister David Cameron to order contingency plans to be drawn up for a no-fly zone.
Since 2004 Libya has been destroying its stocks of chemical weapons. According to the Organisation for the Prohibition of Chemical Weapons (OPCW) it still has 9.5 tonnes of mustard gas hidden in the desert, but it is believed to no longer have the means to deliver it.
Libya, like the other Arab revolutionary republics of Yemen and Iraq, is a country where your tribe can help define your loyalties.
But in recent years the tribal distinctions have blurred, and the country is less tribal now than it was in 1969.
Col Gaddafi himself comes from the Qadhaththa tribe and during his 41 years in power he has appointed many of its members to key positions in his regime, including those for his personal safety.
Just as Saddam Hussein did in Iraq and President Saleh has in Yemen, Col Gaddafi has been adept at playing off one tribe against another, ensuring that no one leader risks posing a threat to his regime.