President Donald Trump has claimed that German Chancellor supported his decision to let his daughter Ivanka briefly sit in for him at a summit of world leaders on Saturday.

Trump took to Twitter to praise his daughter's performance as well as pointing to recent comments by Merkel that acknowledged the procedure was not unconventional.

People reacted with a mixture of fury and bewilderment on social media when a photo of Ivanka sat between UK prime minister Theresa May and Chinese premier Xi Jinping circulated on Saturday.

Today, Trump tweeted: "When I left Conference Room for short meetings with Japan and other countries, I asked Ivanka to hold seat. Very standard. Angela M agrees!"

Merkel had previously told Reuters: "The delegations decide who will sit at the table if the president isn't there, which can happen from time to time. And Ivanka belongs to the U.S. delegation," Merkel told Reuters

"It's well known that she works in the White House and is responsible for several initiatives," she added, pointing out that other delegations do similar things.

Ivanka, along with her husband Jared Kushner, is employed as a presidential aide.

"The very fact that his daughter is senior adviser smacks of the kind of nepotism not seen since John F. Kennedy named Robert F. Kennedy as Attorney General," an unnamed European official told the Washington Post.

The Trump family's attendance at the G20 summit was marred by violent protests. At one point wife and step-mother Melania Trump was unable to get out of her hotel due to the carnage in central Hamburg.

Trump was not content with leaving the matter there, however. Minutes after his first tweet, he sent another, this time directed at Hilary and Chelsea Clinton.

Trump drew a hypothetical comparison between his treatment and the way he believed his former presidential rival Hilary Clinton would have been lauded if she had asked her daughter Chelsea to take up the role.

"If Chelsea Clinton were asked to hold the seat for her mother, as her mother gave our country away, the Fake News would say CHELSEA FOR PRES!" he said.

Chelsea quickly responded to the tweet with a barb of her own, saying: "Good morning Mr. President. It would never have occurred to my mother or my father to ask me. Were you giving our country away? Hoping not."