Washington DC - President Donald Trump on Saturday hailed the US as the "crowning achievement" of human history, and renewed his attacks on political opponents during his 250th anniversary speech.

In a speech delayed by several hours when storms forced the temporary evacuation of crowds in Washington, Trump claimed that under his presidency the US was "prouder than ever before."

While Trump had promised a huge political rally to stamp his brand on the national celebrations, the 80-year-old Republican largely stuck to a more traditionally patriotic script.

"For two and a half centuries, our American republic has stood as the crowning achievement of human history," Trump told tens of thousands of people on the National Mall.

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On stage, he hailed veterans from World War II and the Korean and Vietnam wars. He referred to the latter two as examples of the battle against "communists" he says has resurfaced with recent wins by anti-establishment Democrats.

"Our warriors did not fight communism on battlefields across the word, only to have that menace rear its ugly head right back here in America. We're not going to let it happen," he said. "It's like a cancer, you've got to cut it out."

The comments echoed a similar message to his speech on Friday at Mount Rushmore, during which he warned of a "renewed attack" by "radicals and extremists."

The US leader also used the speech to boast of the recent military campaigns against Iran and Venezuela, claiming Washington had "wiped out" Tehran's military.

The speech was short by Trump's standards, at only around 45 minutes long. It was concluded by a huge fireworks display, which Trump said set a record as the world's largest.