Hundreds of campaigners have demanded protection for services at Prince Philip Hospital in Llanelli in a protest at the Welsh assembly.
They delivered one of the biggest petitions received by the assembly's petitions committee, with more than 26,000 signatures.
Protesters called for Hywel Dda Health Board to retain a major accident and emergency unit at the hospital.
The health minister said the board would unveil proposals in late summer.
The board has put forward options that would turn the A&E unit into an "urgent care centre".
The Welsh government says the NHS must change and has asked health boards to draw up plans that could see some services being centralised.
But ministers insist hospitals will not be closed or downgraded as a result of the reforms.
Buses paid for by Llanelli Town Council took protesters to the Senedd in Cardiff.
Politicians from all parties attended the protest, including the town's Labour AM Keith Davies. His party has accused Plaid Cymru of scaremongering over the future of hospitals.
Bryan Hitchman, chair of the Sosppan (Save Our Services Prince Philip Action Network) campaign group, said: "Common sense must prevail.
"Common sense means treatment near to home.
"We could end up with Llanelli becoming an NHS desert. This is the way things are being eroded away from us."
The Welsh government pointed to a planned £20m investment in the hospital between 2014/16 to develop elective care and treatment services.
About £6m has been spent on the hospital's breast care unit in recent years, a spokesman said.
In the Senedd chamber, Health Minister Lesley Griffiths said: "I don't want to see any downgrading of any services in Wales."
Hywel Dda Health Board was doing all it could and had a prolonged period of engagement, she said
"But of course I can assure everyone in this chamber and outside of it that I understand how strongly people feel about health care services in their local area.
"But this really is not about buildings.
"This is about services and really making sure we have the proper services for the people of Wales."
She added: "We all know the status quo is unsustainable and services have to change."