ZAWIYAH, Libya (Reuters) - At a checkpoint on the road leading south out of Zawiyah, a rebel fighter on Sunday sat at a table with a sheet of paper listing suspected agents working for Libyan leader Muammar Gaddafi.
Anyone passing whose documents matched a name on the list was likely to end up in a makeshift prison attached to a nearby clinic where about 15 government soldiers were already held.
For the rebels who made a dramatic advance to Zawiyah, 50 km (30 miles) west of the Libyan capital, at the weekend, such evidence of organisation could be the difference between ousting Gaddafi and being forced into yet another humiliating retreat.
After a six-month-old conflict when inexperience and ill-discipline undermined their offensives time and again, rebels have formed themselves into a more organised fighting force.
That organisation will be crucial if they are to consolidate their latest advances which allow them to encircle Gaddafi's stronghold in Tripoli and, they hope, force his capitulation.
"The rebels have a reputation for being chronically over-optimistic and they have to be able to hold their gains, which they haven't always succeeded in doing," said Shashank Joshi, an analyst with the Royal United Services Institute in London.
"The way they fight now is going to determine how successful they will be."
The last time the rebels made rapid territorial advances was early in the conflict in the east of Libya. Then rag-tag groups of leaderless volunteers would dash across the desert, only to pull back just as quickly when Gaddafi's troops started firing.
The rebels who took Zawiyah, many of them hardened by months of fighting in the Nafusa mountain range to the south, were a different proposition.
"During the previous uprising the young people like me, we just attacked on our own free will," said Nagi, a 31-year-old rebel fighter from Zawiyah.
"But now when our commanders speak we do as we are told and we have tactics, it is not just a free assault without thinking."
Rebels are now formed into units, mostly based on their native towns or villages, and each unit has a commander. That may be standard battlecraft, but for the sometimes anarchic rebel movement, it is a novelty.
In contrast to previous offensives, the rebels do not waste ammunition. There is little celebratory gunfire.
The rebels also seem to have at least a rudimentary idea about military tactics.
"We are going to go to Tripoli very methodically," said Murad Badda, a 39-year-old shopkeeper-turned rebel fighter who was driving around the town on Monday in a pickup truck with a group of other fighters."
"We will do what we did during our offensive from the Western mountains. We will attack one area, clear it and then move forward very carefully."
There was still some bravado.
"We don't expect a huge amount of resistance. We believe that Gaddafi's men think they're fighting a losing cause. That's why they've been retreating during our offensive," Badda said.
There remains a risk the rebels could be forced to retreat. Pro-Gaddafi forces on the eastern edge of Zawiyah were on Monday firing Russian-made Grad rockets and mortars at rebel positions, though there was no sign they were gaining ground.
But the rebels are better placed than their comrades who were put to flight in eastern Libya a few months back, said John Drake, senior risk consultant with London-based consultancy AKE.
"The difference now ... is that the rebels emerging from the Nafusa mountains appear to have acquired quite a significant amount of weaponry."
"They were struggling a few weeks ago but not only do they now have greater amounts of weaponry, many of them also have quite high morale," he said.
Another element of the rebels' improved battlefield performance is a new ability to coordinate.
The attack on Zawiyah was carried out by a combined force of rebels from the town itself, and from Zintan and other towns in the Western Mountains which in the past have treated each other with suspicion.
While the main force pushed towards Zawiyah, another was attacking Gaddafi forces in Garyan to the southeast and a third was encircling a government garrison in Tiji, to the southwest.
Those operations meant that the forces that could have cut off the rebel supply lines to Zawiyah were pinned down and unable to move.
"We have better cooperation between rebels ... than ever before," said Amin Mustapha, 40, a rebel in Zawiyah. "This time we all feel very united and it will help us when we try to get Tripoli."
The killing last month of Abdel Fattah Younes, the opposition's military commander, was unlikely to have disrupted preparations. Fighters in the west of Libya have a large degree of autonomy from the rebel leadership in the eastern city of Benghazi, where Younes was based.
The coordination most vital to the rebels though is with NATO warplanes. The alliance says it is attacking solely to protect civilians under a U.N. mandate.
"We are not coordinating our strikes with the rebels. We are not clearing the way for the rebel advances. It's the other way around: Gaddafi is moving his forces into the open to respond to rebel advances and we are reacting to that," a NATO official said.
However, along the route that took the rebels from the Western Mountains to Zawiyah was a trail of buildings and tanks which had been destroyed by NATO air strikes. When rebels attacked pro-Gaddafi forces in Garyan on Sunday, NATO struck too, sending plumes of smoke into the air.
"They (NATO) are not picking targets at random, or simply to protect the civilian population, they are picking targets in order to help consolidate rebel gains," analyst Joshi said.
One rebel from the Western Mountains told Reuters a group of intelligence operatives from a NATO member state had set up a coordination centre in the rebel-held town of Nalut, near the border with Tunisia, though it was not possible to confirm that.