Enthusiasts are celebrating the 50th anniversary of the Routemaster bus.
The Routemaster bus is a London icon
Transport for London (TfL) said A "huge party" is taking place in Finsbury Park, north London, to mark RM1's Golden Jubilee.
The free event will also see the launch of the Save The Routemaster campaign which wants to stop the famous bus being phased out of service.
The two-day programme includes a parade of buses and a "pasting a poster on a bus" competition.
TfL says the Routemasters are being taken out of service by the end of 2005, because they are not accessible to all sections of society, such as the disabled and the elderly.
TfL's head of surface transport Peter Hendy said: "London needs a system that truly delivers public transport for everyone and that is the buses' main strength.
"London's buses are nearly 100% accessible and play a vital role in getting people to work and to shops, to the healthcare, education and job opportunities that London provides.
"We can show that not only does this enable greater social inclusion, but also that buses are being increasingly used by people who have access to, but are choosing not to use cars."
But Andrew Morgan, chairman of the Routemaster Owners and Operators Association which has organised the event, said: "It is a great shame that we are losing such a huge icon for London. It is a great loss."
London mayor Ken Livingstone has said some Routemasters will be kept for heritage and tourism purposes.
A free bus service operated by Routemasters will take people to Finsbury Park from Manor House in north London.