Hillary Clinton has said young people in Britain expressed "fear, disgust and a feeling of being let down" after the Brexit referendum.
The former US first lady, who lost the race for the White House to Donald Trump in 2016, said that "millions" of young people around the world "currently feel that democracy isn't working for them".
She received a standing ovation after delivering a University of Oxford Romanes Lecture at the city's Sheldonian Theatre on Monday.
"Young people are proving to be the most effective activists, change-makers and problem solvers anywhere in the world," she said.
She added: "Today in the UK, where nearly three quarters of 18 to 24-year-olds voted to remain in the European Union, one has to ask: could more have done the same, enough to turn the tide?
"Because after the vote, young people spoke out expressing fear, disgust and a feeling of being let down by the older generations."
The former US secretary of state said she is optimistic about the future and called on younger generations to get involved in politics.
"It is my hope that young people here at Oxford will commit to the hard and important work of building a democratic process that represents your hopes, concerns and dreams," she added.