Musk, Bezos and Zuckerberg are the world's three richest people, and in addition to Google co-founder Sergey Brin, who also attended, their combined fortune sits at just above one trillion dollars, according to Forbes.

The 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 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.

TikTok on Sunday credited Trump for promising an executive order to save the app from an American ban, though its fate in the United States remains unclear while under Chinese company ByteDance's ownership, in defiance of a US law.

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.

"They have even better seats than Trump's own cabinet picks. That says it all," said US Senator Elizabeth Warren in a social media post.

Their 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 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 in the balance.

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 cutbacks 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 that could force their breakup.

"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, 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.