Yemen's electricity company said protesting tribesman had blown up main pylons, cutting off power to the capital, Sanaa, and plunging it into darkness.
Power outages are common in Yemen, where the cash-strapped government struggles with resources.
Sabotage attacks on oil pipelines and electricity pylons are also common in the country, which is flush with weapons and where the government has little control outside the main cities.
Security officials said repair teams have not been able to reach the damaged pylons in Marib, north-east of the capital, because the tribesmen were blocking the way.
The officials said the tribesmen are protesting against a death sentence against one of their members convicted of belonging to al Qaida and killing security agents.
Mubarak Hadi al-Shabwani was sentenced to death in 2009 but appealed.
The court endorsed the death sentence on Tuesday, prompting his relatives to blow up the pylons in protest.
The officials said the military is threatening to use force if they do not clear the area.
Sanaa, already short on power, was blacked out by the blasts late on Tuesday. Many in Sanaa rely on generators to make up for power shortages, but there are not enough generators in the impoverished nation.
On Wednesday, the same court sentenced a man to death after he was convicted in the fatal shooting of a French manager of an Austrian oil company in 2010.