A White House official told DailyMail.com that the younger Trump's fill-in role was nothing out of the ordinary.
UK Prime Minister Theresa May was seen leaving the G20 meeting room at one point, a source with knowledge of the summit's goings-on said, and her place was briefly taken by an aide.
But when the 35-year-old former fashion executive sat at the table with Xi Jinping, Recep Tayyip Erdogan, Angela Merkel and Theresa May, Trump's enemies pounced.
This photo, snapped by a Russian official at the G20 summit in Hamburg, Germany, shows the moment when Ivanka Trump stood in for her father and sat with world leaders
US President Donald Trump (R) holds his daughter Ivanka Trump (C) at a panel discussion titled 'Launch Event Women's Entrepreneur Finance Initiative'
Historian Anne Applebaum tweeted a complaint, denouncing 'an unelected, unqualified, unprepared New York socialite' being seen as 'the best person to represent American national interests.'
Brian Klass, an Oxford scholar who once served as deputy campaign manager for a Democratic governor of Minnesota, tweeted that she was representing the U.S. as an 'unelected, unqualified daughter-in-chief.'
'This kind of thing happens all the time. In dictatorships,' added self-described women's rights activist Amy Siskind.
And Charles Blow, the liberal New York Times columnist, wrote flatly: 'Why the hell is Ivanka Trump sitting in for daddy at G20 meetings?! What are her qualifications? Who voted for her?'
It's not uncommon for leaders to send proxies to attend meetings in their steads, even at high-profile international summits.
Hillary Clinton also famously stepped in during the 1990s to lead a government commission on redesigning the nation's health care system, standing in for her husband despite not having been elected to any official post.
The Trump White House official said Saturday that Ms. Trump 'was sitting in the back and then briefly joined the main table when the President had to step out.'
Reactions from Trump critics were fast and furious after the Ivnaka photo went viral