Celebrities including Jada Pinkett Smith, Amy Poehler and Leona Lewis have got behind an online campaign demanding the release of the kidnapped Nigerian schoolgirls.

More than 250 young females have been snatched from northern Nigeria in recent weeks by members of the Boko Haram extremist group, who want to deny them a Western education.

A campaign has been launched via Twitter.com, with the 'hashtag' #BringBackOurGirls, and now several stars are supporting the protest, which demands the girls' release and aims to put pressure on the Nigerian government to take action.

Pinkett Smith urged fans to sign a petition calling for U.S. or global help with the situation. Other stars to back the drive include actresses Alyssa Milano, Queen Latifah and Minnie Driver.

Comedienne Poehler posted a picture of herself holding a piece of paper with the hashtag written on it, and told followers, "Shout! Participate! Show that you care!"

British singer Lewis posted a similar snap, while Tv presenter/model Alexa Chung wrote her message in red lipstick on a mirror.

Singer Wyclef Jean wrote, "Children are the reward of life. Rescue our Nigerian sisters. The world is watching, Make your voice heard. Retweet! #BringBackOurGirls."

Public figures have also joined in the campaign, including U.S. First Lady Michelle Obama, Hillary Clinton, and teenager Malala Yousafzai, who was shot for campaigning for women's rights in Pakistan.