The cause of the Friday evening blackout in Baghdad was not immediately clear.
Mahmoud al-Saaedi, the head of the information office of the Electricity Ministry in Baghdad, said "technical problems" cut off electricity on lines from northern Iraq.
He said there was no sign sabotage was the cause.
Iraqi and US forces have clamped down with intensive security measures to prevent attacks on voters at the polls on Saturday.
The lights went out soon after sunset at a time when fasting ends during the holy month of Ramadan, at around 6pm. The power was still off more than two hours later, ahead of the start of the 10pm curfew.
The Baghdad skyline was black except for pinpoints of light from private generators.
The blackout appeared to have affected much of the Baghdad governorate, an area of about 5600 square km.
"This has affected large areas of the central Iraq and other large surrounding regions," al-Saaedi said.
"Our crews are working on fixing them and the problem could be solved soon and the power will be back gradually"
Electricity Ministry. Baghdad
"Our crews are working on fixing them and the problem could be solved soon and the power will be back gradually."
The frequent power outages that hit Baghdad and other parts of Iraq are a constant cause of complaint by Iraqis.
The southern Iraqi city of Basra was also plunged into darkness on Friday evening when saboteurs struck the electrical power network.
"The high-tension lines were sabotaged, which left Basra without electricity," said an engineer at the Hartha power station just north of Basra.
The engineer, who asked not to be named, gave no further details, except to say that the outage began at 6:30pm (1530 GMT).
Both cities were still without power at 9:10 pm (1610 GMT).
Meanwhile, a fire broke out at a petrochemical plant in Khor al-Zubair, 40km southeast of Basra, according to an official at the Southern Oil Co.
"The causes of the fire have not been determined, and specialised teams are working to control the blaze," the official added.