A total of 2,570 Automatic Number Plate Recognition cameras are planned to be installed across London to monitor the controversial Ultra Low Emission Zone (Ulez) which is due to launch on August 29.
More than 300 have already been set up in the capital’s outer boroughs.
Pictures on social media have now shown that some of the cameras have been covered up.
One showed a box with the words “stop electing idiots” written on it.
Another camera appeared to have been covered up with a Sainsbury’s bag for life.
Mr Khan, London’s mayor, has faced fierce opposition to the expansion of the scheme, which charges drivers of some types of older, more polluting vehicles £12.50 a day to drive in London.
Most petrol vehicles less than 16 years old and diesel vehicles under six years old meet these standards.
One person who shared the pictures online wrote: “Absolutely love that locals in London have covered the money grabbing Ulez cameras with cardboard boxes and bags for life.”
Critics of the expansion, which will cover most of the area within the M25, say it unfairly targets the poor.
Campaigners have also raised concerns about privacy after it was revealed that the Met Police and British Transport Police could have access to the cameras to help them fight crime.
While the cameras have started to go up, they have not done so in Bexley, Bromley, Harrow and Hillingdon where the councils are aiming to challenge the mayor in court over the move.
Although some online commentators were in favour of the expansion, with one Twitter user saying: “Who wants to live in a city full of dirty air?”
A spokesperson for Transport for London (TfL) previously said that it removes any obstructions from its cameras as part of its regular maintenance.
It comes after pictures emerged of a Ulez camera close to Heathrow with its wires cut in Uxbridge.
Another four cameras in Abbey Wood, Greenwich, were also shown to have been vandalised earlier this month.
Analysis recently carried out by the RAC revealed that up to 700,000 motorists could be liable for the £12.50 charge when the scheme is expanded.
TfL and the Mayor’s office though have contested that figure, saying that only one in 10 cars being driven in the outer London “doughnut” area, which is set to come under the scheme, will have to pay the levy.
Earlier this week, Mr Khan said that there had been a marked increase in the number of cars that comply with the Ulez exhaust emission rules, which means they do not have to pay the charge.
Londoners can apply for a means-tested grant of up to £2,000 to scrap their non Ulez-compliant cars or motorcycles.
Mr Khan has also asked TfL to look into using Ulez cameras to charge car users in a “pay-as-you-drive” scheme in the capital.