Two pylons in the province of Parwan were bombed late on Friday, cutting off electricity to the capital and neighbouring provinces.
Eleven provinces in Afghanistan have been facing power cuts, with explosions targeting two transmission towers just west of the capital Kabul.
Saturday's outages affecting millions come ahead of the Eid al Fitr holiday that marks the end of the Muslim holy month of Ramadan.
"The enemies...have blown up two electricity pylons with bombs," Hekmatullah Maiwandi, a spokesperson for the state-run DABS electricity company, said in a video statement.
Five teams from the firm have been deployed to carry out repairs, he added.
"The pylons are installed on top of mountains and our teams are trying to fix them," Maiwandi said.
Temporary repairs would be performed to partially restore power by Saturday night before a full restoration of the towers can be completed in two weeks, he added.
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Police said two suspects have been arrested over the explosions. No further details on the suspects were available.
Many residential buildings and businesses in Kabul, a city of about five million people, booted up private generators to ensure electricity supply ahead of Eid celebrations.
Afghanistan is largely reliant on electricity imported from northern neighbours Uzbekistan and Tajikistan, making cross-country power lines a prime target for armed groups.
The incident comes at a time when Afghanistan is reeling from a series of deadly explosions.
The Daesh terror group has claimed responsibility for several of those attacks targeting the minority Shia and Sufi communities in the last two weeks, killing dozens of civilians.
"Nobody is happy during this Eid as so many families are mourning because of the recent blasts. Now the pylons have been blown up too," Khatera Fakhri, a Kabul resident, told AFP news agency.
"When there is no electricity we can't prepare for Eid. Everything is so difficult."