An air monitoring site at Brixton Road in south London began recording levels of nitrogen dioxide above the threshold early Thursday and continued to do so beyond the 18-hour legal limit, according to data from King's College London.
Nitrogen dioxide is a gas emitted by diesel engines that causes lung disease and respiratory problems.
An environmental activist said an "incredible" level of pollution was detected on Brixton Road -- a busy shopping and transport hub -- and that bus services should have been suspended and pedestrians warned to leave the area.
European Union law stipulates that a maximum nitrogen dioxide concentration of 200 micrograms per cubic meter must not be exceeded for more than 18 hours over the year.
The Brixton Road site measured its 19th hour above the threshold at 9 p.m. Thursday.
Simon Birkett, founder and director of the Clean Air in London campaign, said in a statement: "Hourly nitrogen dioxide concentrations reached an incredible 347.7 micrograms
per cubic (meter) in Brixton Road at 9 p.m.
"When conditions are as bad as they were on Thursday, bus services should be suspended, drivers should be advised not to drive and pedestrians should be warned to avoid the area."
However, Gary Fuller of King's College said the reading was "high but not that incredible." He told CNN: "In 2016 we measured nine hours above 400 micrograms per cubic meter alongside London's roads."
Health concerns among residents
"There is huge concern here. People are becoming increasingly aware that air pollution is a problem," said Caroline Russell, a Green Party spokeswoman on transportation and a member of the London Assembly.
"One woman I spoke to this morning has a son with asthma, and she explained how completely debilitating it is for him," Russell told CNN during a visit Friday to Brixton.
"He went for a walk around the block, got wheezy, and had a serious asthma attack triggered by this polluted air."
Last year, the annual air pollution limit was breached eight days in to 2016 -- on Putney High Street in southwest London, according to London Air, a monitoring network run by King's College.
Most main roads in London breach legal limits
The network said pollution levels had generally improved since the EU limits were introduced in 2010, but the majority of main roads in London still regularly breach legal values for nitrogen dioxide.
The mayor announced plans Friday for 10 low emission bus zones deploying the "greenest" buses on the capital's most polluted routes to cut nitrogen oxide emissions.
"London's toxic air is a national embarrassment, which damages more than its reputation as a modern global city," said Alan Andrews, a Brixton resident who is a lawyer for the activist group ClientEarth
"Mayor Sadiq Khan has put forward some welcome proposals to tackle the problem, including introducing a bigger ultra-low emission zone in 2019 and plans to use the cleanest buses on the most polluted roads. It's essential he delivers on these plans and that the national government fully supports him."
But Ian Colbeck, a professor of environmental science at the University of Essex, said trucks and buses are not the only problem.
He pointed to a report issued this week by the International Council on Clean Transportation showing that emissions of nitrogen oxide from new diesel cars -- which converts into nitrogen dioxide once it comes into the air -- were more than double those from trucks and buses and more than six times the legal limit.