A punishing heatwave threatened America's July 4 celebrations, World Cup matches and power grids as near-record temperatures scorched the eastern United States on Friday.

Cities open cooling centres, extended pool hours and urged residents to conserve electricity

With temperatures forecast to reach up to 42 degrees, the US president told an audience he planned to make a long speech, “just to show that I can do anything”.

High humidity is expected to make conditions feel even hotter