Dubbed Reimagine, a project in London took place over the weekend to highlight the implications of traffic pollution and the future prospect of a city more reliant on public transport and electric power rather than vehicles powered by diesel and petrol.

As an event, it was a small one – barely noticed by millions of Londoners. About 25 kilometers of roads were closed in and around London's financial district, the quietest spot on any weekend. 

Free family events replaced cars and buses for 12 hours, including cycle rides, exercise and fitness activities, children's activities and arts and crafts marquees. Street performers seized the opportunity to entertain, storytellers addressed the crowds and yoga instructors led sessions near the iconic Tower Bridge.

The office of London mayor Sadiq Khan said the aim of Reimagine was to "allow children to play and communities to get together."

However entertaining the activities, the message couldn't have been more serious – traffic pollution is a major concern in London and cities worldwide. The idea of car-free days is gathering momentum in other urban areas.

Government statistics suggest more than two million Londoners currently live in areas with illegally dirty air, including more than 400,000 children. Small children are at particular risk because traffic pollution lingers and intensifies at low levels. Research shows that more than 50 percent of London's toxic air pollution is caused by vehicles.

The event organizers said even a limited car-free day would strengthen a campaign against fossil fuels that is gathering steam across the globe.

It follows the extraordinary climate strikes and protests across the world on Friday, when numbers in many cities far exceeded the expectations of the organizers. And it comes ahead of the United Nations General Assembly in New York this week, when the world's leaders will meet to discuss, among other issues, action against climate change.