In Washington, an Independence Day morning was cancelled "after extensive and careful consideration of the safety of our participants, spectators and staff as the top priority", organisers said.

The intense heat also led to the closure of the fair on the National Mall for several hours on Friday, one day before the holiday. The State Fair reopened at 17:00 local time (21:00 GMT) after conditions improved, organiser Freedom 250 said.

"The safety and wellbeing of our guests, volunteers, performers, vendors and staff is our highest priority," it said.

The DC Fire and EMS Department treated several people for "heat-related illnesses" at the fair on Friday, a spokesperson for the department told the BBC. He said these were caused by "record-breaking temperatures" at the event.

At least 11 people were taken by ambulance from the fair, the spokesperson said, but did not elaborate on whether all had heat-related illnesses.

"It is going to be a very busy weekend," a spokesperson for the DC Fire and EMS Department told the Washington Post.

"We know that there are going to be heat-related illnesses on and off the Mall, and we encourage our residents and visitors to take precautions if you're going to be out in the heat."

Robin Ardito, who attended the fair, said she saw a middle-aged woman who appeared to be suffering from a heat-related illness. The woman was being tended to by fair staff with both hands in buckets of ice, she said.

"It was too hot to be holding an event like this," Ardito added.

Another event in Washington DC was affected when US Capitol Police delayed public entry for Friday evening's outdoor A Capitol Fourth concert from 15:00 to 19:00 local time.

Trump is expected to speak outside at a 4 July celebration on Saturday, despite the predicted high temperatures. The president has maintained that he wanted to gather outside for the celebration.

"I'm gonna make a really long speech just to show that I can do anything," he said.