The Ultra Low Emission Zone (ULEZ) has come into force in central London.
Drivers of older, more polluting vehicles are being charged to enter the congestion zone area at any time.
Transport for London (TfL) hopes the move will reduce the number of polluting cars in the capital, and estimates that about 40,000 vehicles will be affected every day.
Mayor of London Sadiq Khan said it was “important we make progress” in tackling the capital’s toxic air.
Anybody who does not pay the charge will face a fine of £160, although a first offence may result in only a warning letter.
The ULEZ is set to be expanded to cover the entire area between the North and South Circular roads in 2021.
TfL estimates the initial scheme will lead to a reduction in toxic emissions from road transport by about 45% in two years.
Mr Khan said London’s air pollution was a “public health emergency” and it was the “poorest Londoners that suffer the worst quality air”.
However, the Federation of Small Businesses (FSB) said many small firms were “very worried about the future of their businesses” as a result of the “additional cost burden”.
Some drivers have also spoken about their anger that governments had previously recommended buying diesel cars which are now being targeted in particular by the charge.