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".

The Republican mocked his Democratic predecessor, Joe Biden, and aimed fresh criticism at the US Federal Reserve's chairman, while dismissing polls that show his own popularity slipping.

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".

According to Gallup, Trump is the only post-World War Two president to have less than half the public's support after 100 days in office, with an approval rating of 44%.

But the majority of Republican voters still firmly back the president. And the rival Democratic Party is also struggling in polling.

The Democratic National Committee (DNC) said Trump's first 100 days were a "colossal failure".

"Trump is to blame for the fact that life is more expensive, it's harder to retire, and a 'Trump recession' is at our doorstep," the DNC said.

Trump conducted his own informal poll in Tuesday's remarks, asking the crowd for their favourite Biden nicknames. He also mocked his Democratic predecessor's mental agility and even how he appears in a swimsuit, while continuing to insist he was the real victor of the 2020 election, which he lost.

Other targets of his ire included Jerome Powell, head of the US central bank, whom the president said was not doing a good job.

Trump touted progress on immigration encounters at the southern border have plummeted to just over 7,000, down from 140,000 in March of last year.

The White House also said almost 65,700 immigrants had been deported in his term so far, although that is a slower pace than in the last fiscal year when US authorities deported more than 270,000.

Part of the way through his speech Trump screened a video of deportees being expelled from the US and sent to a mega-prison in El Salvador.

His immigration crackdown has faced a flurry of legal challenges, as has his effort to end the automatic granting of citizenship to anyone born on US soil.

During Tuesday's speech he insisted egg prices had declined 87%, a claim contradicted by the latest government price figures.

Inflation, energy prices and mortgage rates have fallen since Trump took office, although unemployment has risen slightly, consumer sentiment has sagged and the stock market was plunged into turmoil by the tariffs.

Before the speech, Joe DeMonaco, who owns a carpentry business in Michigan, said Trump's patchwork of on-again, off-again import taxes were starting to increase prices, which he will have to pass on to his customers.

"I was hoping. . . he would approach things a little bit differently seeing that he's a little seasoned coming into a second term," Mr DeMonaco told the BBC. "But we're just treading water and seeing if things get better from here."

But it's clear that Trump's most steadfast supporters stand by him.

"I'm just thrilled," Teresa Breckinridge, owner of the Silver Skillet Diner in Atlanta, Georgia, told the BBC.

"He's handling things wherever he can, multiple times a day, and he's reporting back to the people. . . I think the tariffs will end up definitely being in our favour."