The outage began at around 2:30 p.m. ET and affected approximately 40,000 Pepco customers in the District at its peak. A few dozen customers in neighboring Prince George's and Montgomery counties in Maryland were also affected, The Washington Post reported.
Police reportedly received a surge of calls from people trapped in elevators as the city sweated out 90-degree temperatures.
The outage affected the neighborhoods of Shaw, Woodley Park, Van Ness, Adams Morgan, DuPont, Logan Circle, U Street, and Mount Pleasant.
"We are working to restore service to customers as quickly and safely as possible and are investigating the cause of this incident,” Pepco tweeted.
Pepco told customers to call the company's outage number at 1-877-737-2662. A website referred by the utility indicated that power would not be restored to all affected residents until early Sunday.
At approximately 5 p.m. local time, District of Columbia Deputy Mayor Kevin Donahue tweeted that approximately 21,000 people remained without power.