Musk, Bezos and Zuckerberg are the world's three richest people, according to Forbes. Google co-founder Sergey Brin, who also attended, ranks seventh.

US tech tycoons have spent the weeks since the election courting favor with Trump, marking a dramatic shift from Silicon Valley's more hostile response to his first term as president four years ago.

Attendees also included Apple CEO Tim Cook and Google CEO Sundar Pichai.

TikTok CEO Shou Chew sat in the back row of the stage, even as his platform's future remains uncertain. Trump later in the day ordered a 75-day pause on enforcing a law that would effectively ban TikTok in the United States.

Despite highly limited seating after the ceremony was moved indoors due to bad weather, Meta CEO Zuckerberg attended with his wife Priscilla Chan, while Amazon executive chairman Bezos was accompanied by his fiancee, Lauren Sanchez.

"When the three wealthiest men in America sit behind Trump at his inauguration, everyone understands that the billionaire class now controls our government," left-wing US Senator Bernie Sanders said in a social media post.

The tech titans' prominent positions on the inauguration stage was particularly notable for Zuckerberg, whom Trump had threatened with life imprisonment just months ago.

The Meta chief recently made headlines by brashly aligning his company's policies with Trump's conservative worldview, notably by eliminating fact-checking in the United States and relaxing hate speech restrictions on Facebook and Instagram.

Musk has shown the strongest support for Trump, spending $277 million to help him and other Republicans win November's election while transforming his X platform into an amplifier for pro-Trump voices.

Bezos, like Zuckerberg and his peers, has visited Trump's Mar-a-Lago estate in Florida leading up to the inauguration, with favorable treatment, government contracts and reduced regulatory scrutiny for Amazon at stake.

As owner of The Washington Post, Bezos sparked controversy by blocking the newspaper's planned endorsement of Democratic Vice President Kamala Harris for the 2024 presidential election, triggering newsroom protests and subscriber cancellations.

Musk has been named a leader of the so-called Department of Government Efficiency to advise the White House on cuts to public spending and has spent much of the past two months at Mar-a-Lago.

- 'Paid access' -

While Musk's SpaceX is already a major government contractor, Amazon's AWS cloud computing division and Google also count the US government among their biggest clients.

Google, Meta, Apple and Amazon are also fighting landmark antitrust lawsuits from the US government.

"These are very wealthy people who have basically paid for access, which is something that they would do for any upcoming administration even if we all recognize Trump is very transactional," said Andrew Selepak, a media professor at the University of Florida.

"They're making sure it's very clear that their faces, names, and especially their money, is here," he added.