Drivers of older, more polluting vehicles now have to pay almost twice as much to drive in central London.
Mayor Sadiq Khan's £10 T-Charge, which mainly applies to diesel and petrol vehicles registered before 2006, has come into force.
It covers the same area as the existing congestion charge zone, bumping up the cost to £21.50 for those affected.
Opponents said the scheme would "disproportionately penalise London's poorest drivers".
The measure is the latest attempt by Mr Khan to improve air quality in the capital and, according to the mayor's office, will affect 34,000 motorists a month.
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Speaking on the Today programme, Mr Khan said: "We've got a health crisis in London caused by the poor quality air.
"Roughly speaking each year more than 9,000 Londoners die prematurely because of the poor quality air - children in our city whose lungs are underdeveloped, with adults who suffer from conditions such as asthma, dementia and strokes directly caused by poor quality air."
However, Simon Birkett, from the campaign group Clean Air London, does not believe the move goes far enough.