Relatives of dementia patients in the Malvern ward at Sutton Hospital took their protest to Sutton High Street on Saturday, collecting 638 signitures.
The ward may close under proposals being considered by Sutton and Merton Primary Care Trust (PCT) to change how services for older people with mental health needs are delivered.
Despite assurances that standards of care would not be compromised, relatives remain unconvinced.
Jim Aberdein, whose mother is in the late stages of Alzheimer's, said he hoped the protest would help.
"Many people who stopped on Saturday had connections to Malvern ward.
"All remembered it with fondness.
"We believe the campaign is having an effect the PCT has extended the consultation period until the end of February.
"But they have yet to give us any reassurance that they won't simply farm out our loved ones to private nursing homes.
"Malvern ward is a vitally important resource for Sutton, it is crazy to even think about closing it down".
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