East London Line workers and supporters have staged a mock funeral in a protest against privatisation.
RMT union members walked by City Hall in a warning over "unnecessary, damaging and potentially dangerous" privatisation of the line.
The line is to close next week for renovations, and when it reopens in 2010 will run by a private company.
A spokesman for Mayor Ken Livingstone said the line's integration with other networks was "a huge step forward".
The line will be run by MTR Laing when it reopens in three years' time.
"Privatisation of the East London Line is overwhelmingly opposed by the public and by the people who get out there and run the network, and it is all the more galling because it is completely unnecessary," RMT general secretary Bob Crow said.
"Beneath all the spin the fact is that direct public control of the East London Line's operations is being surrendered to fat cats whose sole aim is to squeeze as much profit out of it as possible."
He added: "A report from a leading rail safety expert has told us that the fragmented London Rail set-up will make the East London Line more difficult to operate safely."
The mayor's spokesman said: "The extended East London Line and its integration with the North London Line will be a big improvement in public transport serving some of the most deprived parts of London.
"There will be an investment of £1.4bn to improve services, trains, stations and security. Hundreds of more staff will be recruited."
He added: "The extended East London Line will be run to the same exacting standards as the rest of the network, and will be under the control of the Mayor and Transport for London, which will set fares, decide the frequency and nature of the train services."