Malala Yousufzai is fighting for her life in hospital after a masked gunman shot her in the head as she made her way home from school.
The 14-year-old student, who writes a diary about life under the Taliban for Britain's BBC, was targeted by members of the militant group for campaigning for girls' rights to education.
Stars including Fonda and actress Mia Farrow have taken to Twitter.com to praise Yousufzai's courage, urging followers to keep the teen in their thoughts.
The Easy Rider actor writes, ""Keep Malala in your hearts 2day (sic). Make this young woman a hero in the whole world... Keep the fight for justice alive... Think fighting injustice! Keep Malala alive!"" while Farrow tweets, ""Prayers for child hero who dared to speak out for girls education. Now fighting for life after Taliban gunned her down.""
Slumdog Millionaire star Freida Pinto writes, ""Heartbreaking! Please take a moment to recognise Malala's bravery. Praying that she has a quick recovery,"" and Avatar star Rodriguez offers, ""My heart goes out to Malala Yousafzai. It's a shame human evolution has yet to reach a point of gender equality world wide. It's 2012.""
Madonna used her sold-out concert in Los Angeles on Wednesday (10Oct12) to show her support for the teen, writing 'Malala' on her back and dedicating her hit Human Nature to the youngster.
She told the crowd, ""This made me cry. The 14-year-old schoolgirl who wrote a blog about going to school. The Taliban stopped her bus and shot her. Do you realise how sick that is?""