Last year's event attracted thousands to Trafalgar Square

The event marks National Playday which last year attracted 15,000 people to Trafalgar Square.

This year the free event, which runs from 1100 to 1600 BST, is called Activ8 and focuses on physical fitness.

There are zones for the under-5s, teenagers, sports, healthy eating and a "chill-out" zone.

It is all part of mayor Ken Livingstone's strategy to make London a more child-friendly city.

He has published a guide for councils and developers with the aim of creating more play areas for children.

'No play space'

Wednesday's event focuses particularly on fitness and encouraging physical activity.

Activities range from bubble shows for the toddlers to a juice bar and urban dancing for teenagers.

There is art and yoga in the chill-out zone, football, tennis, basketball, bowling and mini golf in the Body Zone and healthy food and cooking demonstrations in the Food Zone.

Launching the guide earlier this summer, London's deputy mayor Nicky Gavron said: "In the past, too many developments have been allowed without play space and London's children are suffering the consequences.