Launching the Meat Free Mondays campaign, McCartney urged people to cut down on meat consumption to help slow global warming.
I think many of us feel helpless in the face of environmental challenges, and it can be hard to know how to sort through the advice about what we can do to make a meaningful contribution to a cleaner, more sustainable, healthier world. Having one designated meat free day a week is actually a meaningful change that everyone can make, that goes to the heart of several important political, environmental and ethical issues all at once.
McCartney was joined by his daughters Stella and Mary for the launch event at St James's Park, London, along with Ono and other guests including Moby, Kate Bosworth, Kelly Osbourne and Lauren Laverne.
He also claimed that reducing meat consumption would help to fight global hunger and improve the welfare of animals.
Give up one day and then it will be two days maybe. It's a very, very intelligent idea.
In 2006 a UN-backed study showed that the livestock industry was responsible for 18% of man's global greenhouse gas emissions, partly because of deforestation in the Amazon.
Meat Free Mondays will be supported by several high-profile chefs, including Giorgio Locatelli and Yotam Ottolenghi, who have created vegetarian recipes for the campaign's website meatfreemondays.co.uk. Linda McCartney Foods is also promoting the message, while Oliver Peyton and other restaurant owners will be highlighting meat-free dishes.
Sir Paul said: "I thought this was a great idea. To just reduce your meat intake maybe by one day a week and this would seriously benefit the planet.
My family have been this way for years - vegetarians, that is.