Afghan rebel saboteurs cut main power transmission lines to Kabul last week, leaving the capital almost totally blacked out for four days, western diplomatic sources reported today.

The sources also said that during one of the most bitterly cold periods of the winter, the Soviet-backed regime of Babrak Karmal ordered 50 percent cutback in gasoline and heating fuel supplies to foreign embassies in Kabul, and that the already long lines at fuel stations had increased.

According to the diplomatic reports, lights all over Kabul went out about 11 p.m. on Feb. 4 after sabotage to four main transmission lines linking the capital to power stations in Naghlu and Sarobi, located between Kabul and Jalalabad.

The only electrical power reaching Kabul, the diplomatic sources said, was from a 15 megawatt diesel generating plant at Pol-e-Charakhi, and it faces problems because of a shortage of diesel fuel. The plant has been able to supply power only to a hospital in the Wazie Akbar Khan district, the president's and prime minister's residences and several main government buildings, the diplomatic sources said. Many Soviet buildings, including the Microrayon housing complex, have been without power, they said.

Western diplomatic reports originating from Kabul generally have been more reliable than the foreign mission accounts of Afghan guerrilla warfare in the remote regions of Afghanistan. The diplomatic sources said that for the first time, Soviet and Afghan government personnel in the capital were experiencing the same hardships of winter as the city's ordinary citizens.

They said that yesterday power was restored to some sections of Kabul, but that the sabotage last week indicated the rebels are now prepared to risk alienating civilians in the capital to attain the psychological advantage of pressing the guerrilla war there.

The diplomatic sources said the Babrak government is having difficulty finding tank truck drivers to make the hazardous run to fuel depots on the Soviet border, and was offering generous incentives for the job.