The first new Routemaster has begun its first day of service in London, Transport for London (TfL) said.
The launch of the new bus, which will run between Victoria Station in central London and Hackney in the east, wasdelayed for a weekby paperwork.
TfL said a software glitch meant the bus had to be run with its distinctive rear platform shut, calling it "teething problems".
The mayor called the bus "stunning" and "tailored to the London passenger".
Following the new driver-and-conductor vehicle was a "protest" bus covered in slogans attacking the rise in public transport fares in London.
Mayor Boris Johnson has been criticised by the Labour, the Lib Dems and Green Party over the cost of the buses.
Mr Johnson announced plans for the new buses, which run on a hybrid diesel-electric motor, in his 2008 election manifesto.
In total, eight buses with an open "hop-on, hop-off" platform at the rear, costing £11.37m, will run on route 38. They will be staffed with conductors and will not run at night or during the weekends.
The last of the popular, open-platform Routemasters was withdrawn from regular service in December 2005, although some still run on tourist routes.
'Expensive bus ticket'
Mike Weston, of TfL, saidhe thought the new buses would become "a much-treasured sight on the streets of London".
But in an open letter to the mayor, Labour MP for Tottenham David Lammy said each new bus costs £1.4m compared with the conventional double-decker bus which costs about £190,000.
"Riding this bus is surely the most expensive bus ticket in history," he said.
"With 62 seats at a cost of £1.4m, the cost per seat is £22,580. At £22,695, you can buy a brand new 3 series BMW."
The Green Party said its London mayoral candidate Jenny Jones had questioned "how the mayor will deal with the problem of fare evasion and also, whether expenditure on the new bus is the best environmental choice".
"Jenny is concerned that London bus operators will refuse to buy these new buses for London, as their costs will be considerably higher if they are unable to re-sell them second hand to either UK operators or foreign operators," a Green Party spokesman said.
Caroline Pidgeon, leader of the Lib Dems on the London Assembly, said: "What Boris Johnson has managed to achieve is to hike up the cost of a single bus fare by 50% and ensure that buses on routes across London are far more crowded.
"A single new bus which has cost Londoners' a fortune will not cover up Boris Johnson's real record on bus services."
But Mr Johnson defended the new bus, saying: "When ordered in greater numbers it will make a significant economic contribution to the manufacturing industries, while also helping deliver a cleaner, greener and more pleasant city."
"It's not just a pretty face," he added.
"The green innards of this red bus mean that it is twice as fuel efficient as a diesel bus and the most environment-friendly of its kind."
TfL's surface transport director Leon Daniels said: "This vehicle really has set a new standard.
"It utilises the latest cutting edge engine technology to deliver phenomenal fuel economy and emission performance."