About 1,000 people marched through the centre of Aberystwyth on Saturday to protest against proposed changes to services at Bronglais Hospital.
Protestors marched through Aberystwyth town centre
Campaigners fear that the hospital could be downgraded if Ceredigion Local Health Board's (LHB) plans go through.
One retired consultant claimed patients would have to travel further, in some cases putting them "at risk".
The LHB said the proposals were about "strengthening links" and "certainly not" about downgrading services.
Two options for the future for acute services in mid and west Wales have been proposed.
If either are implemented, Bronglais would become a "linked district general hospital".
Ceredigion councillor Ceredig Davies said there were fears that acute services would be moved from the area to the "M4 corridor".
He said the Arbed Bronglais Emergency Rescue (aBer) campaign committee was formed five weeks ago to "counteract" the proposals.
Mr Davies said people as far north as Merionydd and as far east as Llanidloes used the services at Bronglais, and there were fears they could become "isolated".
Peter Gardner, who was a consultant obstetrician at Bronglais for 12 years, said there was concern that there would not be an obstetrician to treat pregnant women or a paediatrician to treat children at night.
He said: "Such people would have to transfer to another hospital. This would expose them to really quite severe danger in cases of bleeding injuries, or severe respiratory problems.
"It's also been suggested that at night the operating theatres would close at 10pm.
"Following that, patients that came in requiring treatment would be assessed and then sent on to the southern hospitals, again the feeling is they would be put at risk," he said.
"We feel that they are providing us with a less than comprehensive medical service for this area, which is to the detriment to the people who live here."
Ceredigion LHB said the planned changes did not amount to a downgrading of services.
It said: "Being a linked-district hospital is certainly not about downgrading Bronglais. The word downgrading is not written within the document."
It said the proposals would "strengthen links" between hospitals to "help our local services survive and develop in the future".
Ceredigion & Mid Wales NHS Trust said any changes to the service would only be done for the benefit of patients.
A three-month consultation period on the proposals began on 3 April, and the deadline for public responses has been extended until 28 July.