Hillary Clinton said many "adult male leaders" are afraid of youthful climate activist Greta Thunberg.
The former US Secretary of State spoke with her daughter Chelsea in London to present her book – The Book of Brave Women.
In their book, they tell the stories of the women who inspired them, and the 16-year-old Swede is among them.
Greta was included in the book a year ago when the mother-daughter duo read in front of the Swedish Parliament about their lonely climate strike.
"It moved us so much," said Ms. Clinton on Sunday evening at the Royal Festival Hall of the Southbank Center.
Since then, Greta has sailed across the Atlantic, giving the UN a haunting speech on climate change.
"It was fascinating to see how many adult male executives are afraid of this young 16-year-old girl who talks about the climate change threat," said Ms. Clinton.
The US Democratic presidential candidate of 2016 said that the fact that Greta is a young woman "shakes" the paradigm and "old DNA" that still exists in society.
"It's crazy to think how much that still works," she said.
"You could probably connect some of the people who criticized you (Greta) to a polygraph in social media and other environments and say," Do not you think that's a bit sexist? ".
"And of course they would not say and they could even survive because they are so fascinated by the idea that anything she says before we even get the benefits has to be discredited because she should not say it. "
Chelsea added that people in the US are attacking Greta because they can not attack science around climate change.
"In a broader sense, it is their true clarity and fearlessness, and I think that for many people it is incredibly threatening to be so unfocussed and relentlessly focused on the future," she said.
Other women in the book are the youngest Nobel laureate Malala Yousafzai, the American pedagogue and advocate of the blind and deaf Helen Keller and the professional tennis player Serena Williams.
The book also features other lesser-known women, such as Diana Nyad, who was the first non-cage person to swim in shark and fatal jellyfish waters from Havana, Cuba, to Key West, Florida.
The duo was asked if they had a list of men they consider "courageous".
Mrs. Clinton said her first thought was Nelson Mandela.
"He was one of the more remarkable, gustier, more human leaders and people we know," she said.
"There is a real fight going on today, there is a lot of toxicity coming from male leaders, not to mention it."
She also said former US President Barack Obama is a role model.
"I think Barack Obama was a very brave person, but he worked so hard not to show it," she said.
"He knew the undercurrents that worked against him, and he has superbly defended women's rights and opportunities."