Drivers from the minicab firm Addison Lee have staged a protest over reduced rates introduced by the company.
Their black people carriers filled the famous square, hooting horns and hampering traffic.
London Live spoke to some of the drivers and representatives from the GMB Union, which was leading the action.
In a statement, Addison Lee denied that the recently introduced changes had adversely affected drivers.
"Despite the number of private hire drivers in the London market increasing by over 50% in the past 2 years, our average driver earnings have increased by 5% compared to this time last year," said a spokesperson.
"We can assure all our clients that our drivers are fairly compensated for their time and service."