A dangerous heat wave that has lasted for several days continued to intensify across the eastern US on Friday, with the Centers for Disease Control and Prevention (CDC) reporting "extremely high" rates of heat-related emergency room visits in parts of the country.

The heat wave shattered temperature records, led to more emergency room visits, and created serious risks for millions of people preparing to spend the Fourth of July outdoors.

More than a dozen places in the Mid-Atlantic and Northeast either matched or broke their record high temperatures for Friday, including Washington, DC.

The city reached 39C (102F), exceeding the previous record of 38C (101F) set in 1872.

By Saturday, the worst of the heat is expected to move slightly south, though Washington, DC, is still forecast to reach 39C (102F).

That would make it the hottest Fourth of July ever recorded in the city.

Philadelphia and New York City are also expected to approach 38C (100F), with heat index values near 41C (105F).

Several holiday events were canceled or postponed because of the extreme heat.

Washington, DC, canceled its Fourth of July parade planned for Saturday morning, while President Donald Trump's Great American State Fair delayed its opening by two hours, moving it to noon.

"Despite the heat, which isn't as bad as predicted, the crowds in D.C. are INCREDIBLE! The love of our Country has never been stronger! The Air Shows are at a level never seen before - What great pilots, what great equipment!!!" Trump said on Truth Social.

"The Reflecting Pool looks great, despite all it went through with the Vandal Thugs. Will quickly drain and fix the damage done right after this big weekend. Happy Independence Day. Our Country is Stronger than EVER," he added.

Philadelphia also called off its Friday Independence Day parade, which had been expected to be among the largest in the country over the holiday weekend, with participants from all 50 states.

In the New York metro area, thousands of Con Edison customers lost power on Friday.

The utility company said some outages were intentional shutdowns meant to prevent longer-lasting disruptions. Earlier in the day, voltage reductions were also reported in parts of Staten Island, Queens, the Bronx, Brooklyn and Westchester, according to Con Edison spokesperson Jamie McShane.

A World Weather Attribution analysis found that the combination of extreme heat and humidity seen this week would have been "virtually impossible" without the influence of fossil fuel pollution.

At least 25 dead as heat wave lingers, storms threaten flooding across East Coast

At least 25 people have died in a weeklong heat wave in the US, NBC News reported Sunday, as 40 million people remained under heat alerts across the East Coast, southeast and southwest.

According to state and local officials, New Jersey has reported 22 suspected heat-related deaths, while Illinois reported one fatality and Mississippi reported two.

The National Weather Service (NWS) warned that thunderstorms could bring damaging winds, hail and localized flash flooding across parts of the East Coast through Monday.

Flood alerts remain in effect for 34 million people from Delaware to Connecticut as well as New York City, where forecasters expect up to 3 inches (7.6 centimeters) of rainfall.

Severe storms have also caused power outages affecting hundreds of thousands of customers across several eastern states.

The NWS said heat alerts would continue into Sunday evening across the East Coast, with heat index values — which reflect how hot it feels when humidity is combined with air temperature — expected to reach between 100F (37.7C) and 105F (40.5C) in Philadelphia, Washington DC, Baltimore, Raleigh, Charleston, South Carolina and Jacksonville, Florida.

Forecasters said temperatures across much of the East Coast are expected to gradually ease this week, with daytime highs generally ranging from the 70s to the low 90s.

However, dangerously hot conditions are forecast to persist through midweek in parts of the country. Extreme heat watches have been issued for parts of California and Arizona, including Phoenix and Tucson, from Tuesday through Thursday, with daytime highs expected to reach as high as 114F (45.5C).

In New York City, more than 378 people have visited emergency rooms for heat-related illnesses, according to the city's health department.