Ivanka Trump briefly sat in for her father, President Trump, during a session on Saturday at the Group of 20 summit meeting in Hamburg, Germany, prompting an outcry on social media and a swift defense from her brother Donald Trump Jr.

Ms. Trump, who had been sitting in the back of the room during a discussion of topics that were relevant to her, joined the main table when Mr. Trump had to step out, a White House official said.

When other leaders left the table, their seats were also filled by others in their delegations, the official added.