A rally is being held in support of workers on Southern Railway involved in a long-running dispute over changes to the role of conductors.
Members of the Rail, Maritime and Transport union will strike for 48 hours from Friday, with further walkouts planned in the coming weeks, causing travel misery for hundreds of thousands of passengers.
A protest will be held in Westminster today, followed by a rally at Parliament, to be addressed by union leaders, politicians and transport campaigners.
RMT leader Mick Cash said: "The Transport Select Committee has exposed the ongoing shambles that is Southern Rail and the blind eye that has been turned by the Government to gross mismanagement of this franchise on an industrial scale.
"The select committee called on the Government to engage in talks with the company and the union. That call has been ignored.
"The demonstration and rally will be a chance to show not only RMT's opposition to Southern's profit-motivated drive to get rid of guards on trains, but also to hear from those who will be most affected by the proposed cuts including pensioners and the disabled."
Southern says moving conductors to a new role of on-board supervisor will improve services to passengers, with responsibility for closing train doors switching to drivers.
Southern services have been disrupted for months because of the dispute, staff shortages and other problems.