A dangerous heat wave upended Fourth of July celebrations across swaths of the central and eastern US, forcing officials in the nation's capital and elsewhere to cancel ‌or postpone dozens of parades, concerts and fireworks displays.

Among the events disrupted by the sweltering heat was the Great American State Fair on the National Mall in Washington, a centerpiece of President Donald Trump's efforts to mark the nation's 250th birthday.

The fair, designed to showcase all 50 states, was temporarily closed on Friday afternoon as temperatures reached 101 degrees Fahrenheit (38 degrees Celsius).

Late on Friday, organisers of the National Park Service's Independence Day Parade in Washington announced they had cancelled the annual event due to safety concerns.

The parade had been scheduled to start at 10.30 ET (1430 GMT) on Saturday, with the Weather Service projecting ⁠heat index values to reach as high as 115 degrees Fahrenheit.

Record-breaking temperatures spread to the eastern US from the Midwest earlier this week due to the emergence of a high-pressure system known as a "heat dome," which traps a mass of hot air over a region and can cause humidity to spike.

The scorching conditions also strained power grids. PJM, the largest US power grid operator serving 67 million people in the Mid-Atlantic, South and Washington, D.C., areas, ordered customers enrolled in emergency conservation programs to curb usage.

The steps were needed to battle generator outages, overloaded transmission lines and a surge in air-conditioning demand due to the heat.

In New York, Con Edison said roughly 17,000 customers were without power as of late Friday afternoon. As demand surged, it urged customers in New York City and Westchester County to conserve electricity.

-Reuters