India, July 3 -- The United States is under its highest level of security alert this Fourth of July weekend as hundreds of thousands of visitors converge on the National Mall for a fireworks display billed as the largest in American history, even as a dangerous heat wave threatens to complicate an already massive security and logistics operation surrounding President Donald Trump's marquee celebration of the country's 250th anniversary.

Federal officials designated Saturday's festivities a National Special Security Event, the highest possible security classification typically reserved for presidential inaugurations and Super Bowls, placing the US Secret Service in overall command of the operation.

Tara McLeese, special agent in charge o...