London's congestion charge zone roughly doubled in size at 0700 GMT with a westward expansion coming into force.
Some campaigners and politicians are against the move
The £8-a-day road toll scheme now takes in most of Westminster and Kensington and Chelsea in west London.
Transport for London (TfL) said the first morning rush hour had gone well, but the AA said half-term holidays had reduced the number of cars.
About 55,000 residents in the new zone will receive a 90% discount on the fee - sparking a fear of more congestion.
The daily time frame for the entire congestion charge area also changes - it will end 30 minutes earlier at 1800.
West London residents staged a peaceful demonstration against the new charge.
They said it will damage businesses and cost residents hundreds of pounds a year.
According to TfL, traffic in the original central congestion charge zone has fallen 20% since the scheme began in 2003. It expects a further fall of 15% when the new charge kicks in.
It also said the toll has helped fight pollution, reduce road injuries, increased the number of cyclists and caused more people to turn to public transport.
Mayor of London Ken Livingstone said: "Today's morning rush hour has seen a successful start to the congestion charge western extension.
"Traffic is flowing freely inside the extended zone, on its boundaries and the through route.
"The zone was, until now, one of the most congested areas in the UK and first indications are that traffic levels have been reduced as expected by the scheme.
"London is again taking the lead in tackling the problem of traffic congestion and emissions which blight virtually every major city in the world."
Paul Watters, head of roads policy for the AA, said the rush hour had been generally quiet across the newly-extended zone.
"The free route has been very busy, as expected," he said.
"It is half-term in many parts of the home counties and that will have a reducing effect.
"In addition a lot of people are nervous of using the system, as happened when it was first brought in.
"I think it won't hit home for another couple of weeks."
Angie Bray, of the London Assembly Conservatives, said: "Fewer cars are using the roads yet congestion increases.
"The reasons are poor planning and use of road space. Bus and cycle lanes in conjunction with re-phased traffic lights are making London travel slower."
The city's transport watchdog, London Travelwatch, has supported the congestion charge and the expansion.
But chairman Brian Cooke said a two-zone system would have been better than having one large congestion charge area.
He said: "The inclusion of more residents in the discount scheme will mean increased congestion in the existing zone, thereby lessening the benefits seen over the last few years from the congestion charge."
Darren Johnson, leader of the London Assembly's Green Party, said: "The westward extension of the congestion charge will mean more people enjoy the benefit of safer, less polluted streets and more efficient bus services."
A spokesman for the National Alliance Against Tolls said: "This extension of the charge zone is a lose-lose situation for Londoners.
"It will increase congestion everywhere else including the existing zone."