Rebels have seized most of eastern Libya, the main oil producing region in the OPEC member, from Libyan leader Muammar Gaddafi. But many oil facilities are idle or working at well below capacity.
Some rebels commandeered vehicles from the Harouge facility in Ras Lanuf.
"We have a right to these cars and more. Gaddafi stole from the people and now the people are taking it back," said one armed looter, Nasr al-Abdili, who was taking a pick-up truck.
Oil sources said refining operations and exports of crude by firms operating in the town, including units or ventures with the state-owned National Oil Corp (NOC), had ground to a halt over previous days because of unrest and supply problems.
One man in civilian clothes, who refused to comment, was in a security booth at the Harouge site. He was not stopping anyone from entering. One worker said the firm was usually staffed round the clock but fighting had driven employees away.
Inside the buildings, computers were switched on and a coffee maker was still running, but no one was about. Outside, four people, including Abdili, helped themselves to vehicles. They took two pick ups and a mechanical digger.
An oil worker, who asked not to be named, said of the looting: "I work here. It's a shame what's happened, this is the Libyan people's."
"They've stolen more than 300 4x4s and broken into some offices. Usually there's lots of people here, a 24-hour operation. They're not here because of the fighting," he said.
Rebels were seen driving oil company vehicles around the town of Ras Lanuf.
Rebels now hold an area west of Ras Lanuf that includes al-Sidrah, the last major oil terminal town in the east of the country. That would mean rebels now have all the main oil terminals in the east of Libya in their hands.
On Saturday, a Reuters correspondent saw rebels out in force on the streets of Ras Lanuf and manning checkpoints there. Further west, including the road next to al-Sidrah, there was no sign of either rebels or Gaddafi forces.
Some rebels said advance parties had moved further west but forces were now massing in Ras Lanuf to press ahead together.
At another oil firm in Ras Lanuf, Libyan Emirates Oil Refining Company (LERCO), concrete barricades had been put up across the entrance. "There are no engineers because there's a holiday," said a LERCO security guard who declined to be named.
Two oil sources in Ras Lanuf, who asked not to be identified, said there were two refining firms operating in the town, LERCO and Ras Lanuf Oil and Gas Processing Company (RASCO), a subsidiary of NOC.
They said RASCO had stopped operations for the past week or 10 days and LERCO had stopped work for about three days because of a lack of crude supply to their installations.
The sources said Harrouge and another Ras Lanuf exporter have not been exporting for several days.
On the walls of Harouge oil firm were diagrams detailing its operations, including one showing 13 storage tanks of 500,000 barrels capacity as well as four tanker loading bays.
"Harouge Oil Operations Libya undertakes operation, production and shipping activities," a sign on the wall read.
Harouge is a joint venture between NOC and Petro-Canada, which has a 49 percent stake in the firm Petro-Canada also has a stake in the Ras Lanuf export terminal.
OPEC member Libya usually produces 1.6 million barrels of oil per day, but output has been slashed.
An official working for the eastern oil firm Arabian Gulf Oil Co (Agoco) said on March 1 that output from fields the company operates was down by to a third the normal level.
In normal production times, Libya is the 12th largest exporter in the world and the third largest producer in Africa. It also holds the continent's largest crude oil reserves.