The ad went online on the evening of 20 March on YouTube. It is also being hosted on a Stonewall website and will be shown at the British Film Institute’s Flare (lesbian, gay, bisexual and transgender) festival on 25 March.
The spot, called "no bystanders", opens in a school playground where a chain of insults begins with one child calling another a "ginger". The chain then progresses out of the school gates and into the streets, where adults trade bigoted and racist remarks.
Towards the end of the spot, McKellen, who provides the voiceover, says: "What we learn as children can last a lifetime". The spot aims to tackle abusive language of all kinds, and repositions Stonewall as a charity that fights all types of abuse, not just LGBT issues.
The Gate created the spot, which was written by Robert DeCleyn, art directed by Simone Micheli, and directed by Joanna Bailey through Bare Films. The agency has worked with Stonewall for the past two years and landed the account after a personal recommendation.
McKellen, who helped establish Stonewall in 1989, said: "Abuse ruins people’s lives. We all have a responsibility to take a stand and put a stop to it. By making sure that bullying is reported and prejudice is challenged, we can help ensure that every person in Britain lives free from fear of persecution and violence."
Ruth Hunt, the acting chief executive of Stonewall, added: "This powerful film starkly shows how easily prejudice and bullying can escalate from playground teasing to grown-up violence. No one can afford to be a bystander to this bullying and that is why we are calling on people to make the pledge: hear it, stop it."