A speed limit of 20mph has been imposed on all central London roads managed by Transport for London (TfL) in a bid to reduce road deaths.
All TfL roads within the Congestion Charging Zone (CCZ) are subject to the limit since 00:01 GMT on Monday.
Some critics said the speed limit would make no difference because traffic already moves so slowly in the area.
The Vision Zero scheme is a collaboration between the London mayor, TfL and the Metropolitan Police.
Limits have been placed on roads in Millbank, Victoria Embankment and Borough High Street following strong public support for the proposals.
New signage and road markings with raised pedestrian crossings are also being installed in locations where a high number of people walk, including near the Embankment and Tower Hill Tube stations, TfL said.
A study found that about four lives a year were saved when a 20mph scheme was rolled out in Bristol, but similar plans in Manchester were reviewed over concerns they were not reducing the number of accidents.
TfL-managed roads with the new 20mph limit
- Albert Embankment
- Lambeth Palace Road and Lambeth Bridge
- Victoria Embankment
- Upper Thames Street, Lower Thames Street, Byward Street and Tower Hill
- Borough High Street and Great Dover Street
- Blackfriars Road
- Part of Druid Street, Crucifix Lane and part of Bermondsey Street
- Part of Tooley Street, Duke Street Hill and part of Queen Elizabeth Street
Steve McNamara from the Licensed Taxi Drivers Association said the limit would make little difference.
"Most people would love to go as fast as 20mph in the middle of London," he said.
"Average traffic speeds are 6mph in the centre. I can't think of anywhere in central London it would make any difference at all, at least in the daytime. Those roads are all at a virtual standstill."
Mayor of London Sadiq Khan said: "I am absolutely determined to do everything I can to eradicate all deaths and serious injuries on London's roads and these new measures are a vital step along the way to helping us to achieve this."
He added he hoped it would also encourage more people to walk and cycle in London.