A new rail service, known as London Overground (LO), has taken over the suburban routes operated by Silverlink.
London Overground line will take over routes from Silverlink
Transport for London (TfL) will own the stations and trains but services will be run by London Overground Rail Operations Limited (Lorol).
Lorol, which has a seven-year contract with TfL, is jointly owned by Hong Kong firm MTR and construction firm Laing.
The routes covered are the North London Line, the West London Line, Gospel Oak to Barking and Euston to Watford.
Pay-as-you-go Oyster cards will be accepted on all the routes.
The services will be formally launched on Monday, although Lorol took over the operations at 0200 GMT on Sunday.
'Decades of neglect'
A new fleet of 44 trains will be introduced on the routes replacing the Silverlink trains from the end of 2008.
A TfL spokesman said: "Passengers will notice immediate improvements from day one with the introduction of Oyster pay-as-you-go facilities and stations staffed at all times trains are running.
"However, this railway network has suffered from decades of neglect and our £400m programme of station upgrades, infrastructure improvements and the introduction of the new train fleet will take time to put into effect."
The revamped East London Line, which will run from Dalston Junction to West Croydon, Crystal Palace and New Cross, will join LO service in 2010.
In 2011 the line will be connected to the North London line at Highbury and Islington, creating the first step towards an orbital service.
Brain Cooke, chairman of the passenger watchdog London TravelWatch said: "We welcome the immediate introduction of Oyster pay-as-you-go (cards) on this route, as a real welcome move towards integrated ticketing in the capital."
But Rail, Maritime and Transport Workers union (RMT) said safety of passengers could be compromised.
RMT general secretary Bob Crow said: "We believe that safety will be undermined by unnecessary fragmentation, with two organisations responsible for signalling, two for infrastructure maintenance, two for infrastructure renewals, one for train and station operations and another for train maintenance."