Mothers and babies joined in the protest, organised by Ealing Save Our NHS Campaign, on Wednesday (March 18).
Campaign Secretary Eve Turner said: "Our maternity unit is deservedly much-loved by mums and, as well as having a good reputation, it's safety standards are officially recorded as better than the alternative hospitals.
"It's well-used and much-needed.
"It wasn't long ago it was provided with a new birthing centre at a cost of £2m. The plans to close it are ridiculous."
Campaigners believe the real reason the maternity unit is under threat is so the hospital A&E can be downgraded.
Ms Turner said: "Without a proper A&E you can't have a maternity unit."
Under the Shaping A Healthier Future programme, Ealing Hospital would eventually lose 300 beds and the A&E would be downgraded or closed, say campaigners.
Ms Turner added: "We need a full A&E department at Ealing Hospital as well as a maternity unit. These closure plans should be consigned to the bin."
Dr Mohini Parmar, Ealing GP and Chair of NHS Ealing Clinical Commissioning Group, said: “Midwives and clinicians are clear the changes to maternity services in north-west London will improve healthcare for women by providing benefits such as more senior consultant presence on the labour ward and better midwife-to-birth ratios.
“Additionally, while Ealing Hospital maternity unit is currently a safe place for women to give birth, the standards for maternity units are changing and we know that in future Ealing may struggle to meet those standards. This could lead to an unplanned closure of the unit which could increase clinical risk for women and is not fair on women or staff.
“A significant amount of work has already been undertaken to progress these improvement plans and the assurance of this work is under way. However, further work needs to be done on the operational readiness of the new model of care before a decision on timing of the closure of Ealing maternity unit is made.
“We will continue to work with NHS England to ensure a safe decision about the timing of these changes can be made as soon as possible, while we are also working with midwives at Ealing Hospital on a package of support during this time.”