Members of the the United Cabbies Group (UCG) focused their demonstration outside Transport for London's (TfL) headquarters in Victoria Street.
Traffic built up quickly and the knock-on effect was felt by road users throughout neighbouring areas.
TfL said it was working to clamp down on illegal minicab activity.
It added that 331 private hire drivers had been reported since the recent launch of a high-profile campaign to ensure everyone complied with the regulations.
However, UCG drivers complained that TfL is failing to properly regulate private hire drivers and minicab offices by not carrying out sufficient background checks on people applying for licences.
They also said minicabs, which are legally only allowed to take passengers that have pre-booked cabs, are picking up passengers from the street.
The situation has got worse since the launch of Uber, a smartphone cab booking app, according to the union.
Len Martin, chairman of the UCG, said: "We have continually complained of a lack of enforcement creating a real and present threat to lone females at night."
David Garness, from Dagenham, Essex, who took part in the protest said: "We don't like demonstrating like this but TfL is turning a blind eye to what is going on.
"We have illegal operators coming down to London, sleeping in their vehicles, operating without insurance and without the necessary checks."
TfL said it had urged the UCG to call off the protest saying it had launched an operation to clamp down and disrupt illegal activity by minicab and taxis.
Garrett Emmerson, of TfL, said: "We are determined to protect the livelihoods of all legitimate taxi and private hire drivers through robust enforcement action."
Parts of central London were brought to a standstill by taxi drivers protesting against operators they claim are working illegally.