Boris Johnson launches 2-day Big Draw festival at London Bridge

Top artists and cartoonists, architects and archaeologists will join Londoners for the two-day celebration of London's changing river and skyline.

40 free activities are taking place at 12 London Bridge locations, including Southwark Cathedral, by the Shard, Hay's Galleria, Potters Fields Park, Jerwood Space, HMS Belfast, The Scoop at More London, the Unicorn Theatre, Shunt and Old Guy's House.

"Drawing is not about being the best it's about putting across the way you feel, right there, in the moment," says Boris Johnson.

"The world is your sketchbook and I hope everyone will be inspired to join in The Big Draw."

City Hall was hosting a preview of the Battle of the Cartoonists which takes place on Saturday afternoon.

The Mayor joined cartoonist of the year Peter Brookes of The Times and award winning Steve Bell (The Guardian), as well as 'Tamara Drewe' creator Posy Simmonds, Charles Peattie ('Alex', Daily Telegraph), Tim Sanders (The Independent) and Michael Heath (The Spectator).

With easels at the ready, four national newspaper teams will compete to make the most spectacular banner in The Scoop on Saturday at 3.15pm.

At Friday's preview the Mayor, who despite being a published illustrator claimed he "can't draw for toffee", was persuaded to draw his own doodle.

Sue Grayson Ford, director of the Campaign for Drawing, said: "Big Draw events are designed to break down the 'I can't draw' barrier. Our aim is to show that drawing is a vital tool for learning, observation, thought, creativity and communication. Best of all it's accessible – at the end of our fingertips."

• Find out more about Make Your Mark on the Future and other events happening in SE1 as part of The Big Draw.