Donald Trump targeted opponents and touted a huge AI project on Tuesday in a shock-and-awe start to his second presidency - but faced defiance including a rare public dressing down from a bishop.

The Republican also defended his sweeping pardons of US Capitol rioters, including key figures from the far-right Proud Boys and Oath Keepers groups who were released from jail on Tuesday.

Trump has vowed a "new golden age" for America, signing a slew of executive orders in his first 24 hours on immigration, gender and climate change that overturn many of Democrat Joe Biden's policies.

Flanked at the White House by the chiefs of Japanese giant Softbank, Oracle and ChatGPT-maker OpenAI, Trump announced a venture called "Stargate" which will "invest $500 billion, at least," in AI infrastructure in the US.

"This monumental undertaking is a resounding declaration of confidence in America's potential," said Trump.

Tech barons have swung behind Trump, with the world's richest man Elon Musk even joining his administration. Trump said he was open to Musk buying the Chinese-owned app TikTok to keep it open in the US.

But Trump - at 78 the oldest person ever to be sworn in as president - has also promised retribution as part of what he says is a bid to overhaul Biden's "deep state".

His administration fired Coast Guard chief Linda Fagan - the first woman to lead a US military service - with an official blaming her "leadership deficiencies" and an "excessive focus" on diversity programmes.

'Have mercy'

Trump also withdrew Secret Service protection for former US national security advisor John Bolton, the target of an alleged Iranian assassination plot, with whom he fell out.

"He was a very dumb person," said Trump.

Trump earlier announced plans to fire some 1,000 opponents in federal roles. Four people had already been "FIRED!" he wrote, including retired general Mark Milley, his former chairman of the joint chiefs of staff, who became a prominent critic.

Trump, however, faced fresh criticism from an unexpected and powerful voice on Tuesday when a Washington bishop told him from the pulpit that he was sowing fear among America's immigrants and LGBTQ people.

"I ask you to have mercy, Mr President," the Washington National Cathedral's Mariann Edgar Budde told an unsmiling Trump, seated in the front pew for the customary inaugural service next to his wife Melania.

Asked later what he thought about the remarks, Trump said: "I didn't think it was a good service."

Trump issued measures on Monday to suspend the arrival of asylum-seekers and expel migrants in the country illegally. He also decreed that only two sexes - male and female, but not transgender - will be recognised.

'Ridiculous sentences'

He also granted pardons to more than 1,500 people who stormed the Capitol on January 6, 2021, including those convicted of assaulting police officers.

Two prominent rioters had their sentences commuted: Enrique Tarrio, the former leader of the far-right Proud Boys, and Stewart Rhodes, the head of another such group, the Oath Keepers.

"I thought their sentences were ridiculous and excessive," Trump told reporters.

Trump had infamously told the Proud Boys to "stand back and stand by" when asked during a debate with Biden in 2020 whether he condemned white supremacist and militia groups.

Democrats condemned the "shameful" pardons.

The Republican president meanwhile faced pushback on his order revoking birthright citizenship -- guaranteed by the US Constitution - with 22 Democratic-leaning states launching legal action against the plan.

It would prevent the federal government from issuing passports or citizenship certificates to children whose parents are in the country illegally or temporarily.

Trump is pushing a turbocharged agenda after his inauguration on Monday, in which he gave a speech that mixed dark imagery about a failing America with promises of renewal.

He is also sowing fresh disruption on the international stage.

Trump threatened tariffs against the European Union on Tuesday, adding the bloc to Canada and Mexico as potential targets.

He added that Russia was likely to face fresh sanctions if it did not agree to a peace deal in Ukraine.

AFP