Among the events disrupted by the sweltering heat was the Great American State Fair on the National Mall in Washington, a centrepiece 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 as temperatures reached 38 degrees. Local media said the event later reopened.
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 DC, 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.
In New York City, shoe soles stuck to Manhattan streets. The city's extreme heat warning did not stop fans from gathering on sweltering sidewalks to watch celebrity guests arrive at Madison Square Garden for what was widely expected to be the wedding celebration of pop megastar Taylor Swift and football hero Travis Kelce.
Over 185 million people, more than half the US population, were under heat alerts on Friday, the National Weather Service said. Peak heat indexes could reach 46 C in parts of the country, with several cities setting record temperature highs.
Forecasters and government officials have warned that the heat wave could prove deadly. They urged Americans spending the long weekend outdoors to stay hydrated, seek shade and watch for signs of heat illness at gatherings and public events.
Event cancellations were reported across the Eastern Seaboard for the July 4 weekend, when Americans typically gather for barbecues, parades and fireworks displays to celebrate the nation's 1776 Declaration of Independence from Britain.
