Health service union GMB members will join north-west London residents who plan to turn out in force at twin marches on Saturday in defence of local accident and emergency departments.
Nine major hospitals face a big shake-up under cost-cutting plans that campaigners warn will mean longer journeys for the seriously ill and people who find it hard to get around.
Marchers will begin assembling at 10am in Southall Park and 11am in Acton Park before heading to a rally in Ealing Common set to kick off at 1pm.
GMB regional organiser Tony Hughes said: "If these proposals go ahead, some 700,000 people who live and work in these parts of London will see a dramatic change to the NHS services provided.
"This will impact on the public in longer waiting time for ambulances, longer waiting times at A&E departments and, with the closure of maternity and intensive care units, it is only a matter of time before lives are lost."