Campaign group Doctors Against Diesel held a protest in London, claiming 9,400 Londoners die prematurely each year as a result of breathing in toxic diesel fumes.
Paris, Madrid, Mexico City and Athens have all committed to banning diesel vehicles by 2025.
A spokesman for the mayor said Khan has no legal powers to ban cars in the city.
"The mayor is delivering hard-hitting measures to rid London of the dirtiest diesel cars long before 2025. His immediate plans include charges for the most polluting diesel cars in central London from 2017 and bringing forward and extending the Ultra-Low Emission Zone which removes all but the newest Euro 6 diesel vehicles from as early as 2019."
Saturday's protest came as cars were banned and public transport made free in Paris after the city saw its worst winter pollution levels in a decade.
