US President Donald Trump has celebrated the 100th day of his second term in office with a campaign-style speech, touting his achievements and targeting political foes.

Hailing what he called a "revolution of common sense", he told a crowd of supporters in Michigan that he was using his presidency to deliver "profound change".

Trump has delivered a dramatic fall in the number of migrants crossing illegally into the US, but the economy is a potential political vulnerability as he wages a global trade war.

"We've just gotten started, you haven't seen anything yet," Trump told the crowd on Tuesday in a suburb of Detroit.

Speaking at the hub of America's automative industry, Trump said car firms were "lining up" to open new manufacturing plants in the Midwestern state.

Earlier in the day he softened a key element of his economic plan - tariffs on the import of foreign cars and car parts - after US car-makers warned of the danger of rising prices.

At his rally, Trump also said opinion polls indicating his popularity had fallen were "fake".

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