Cab drivers have protested against plans to give help to women and ethnic minorities entering the profession.
Demonstrators say the scheme allows unfair preferential treatment
They said moves to offer language, numeracy and childcare support for those taking the Knowledge exam was preferential treatment.
But supporters of the scheme, supported by the London Development Agency (LDA), said greater diversity was in the long-term interests of the profession.
Members of the London Cab Drivers' Club (LCDC) demonstrated outside the LDA.
The Knowledge, introduced in 1865, involves remembering every street within six miles of Charing Cross and takes up to three years to attain.
"Most people have had to do the Knowledge in their own way," said Alan Fleming, from the LCDC.
"A lot of people have actually done it while looking after a family or paying their mortgage. I think it's quite an unfair system that certain parties should be singled out for preferential treatment which is what this is all about."
Mr Fleming also labelled the scheme "patronising" towards the groups it was actually trying to target.
Official figures show that only 5% of London's black cab drivers are from ethnic minority communities, and 1.6% are women.
Mayor of London, Ken Livingstone, accused the LCDC of demonstrating a "consistently backward and short-sighted approach" to improving the taxi trade.
"The project simply aims to support people from communities that are under-represented in the industry, as they undertake the arduous Knowledge process, so that over time London will have a taxi trade that looks more like its population," he said.
"This will make London an even more attractive place to companies around the world whose business we wish to attract."
He insisted there would be no compromise over the knowledge qualification process which would remain vigorous.