Campaigners opposed to changes at Prince Philip Hospital in Llanelli have staged what they call a "stop the closure" march and rally.
Llanelli Trades Council, which organised Saturday's protest, says it fears the hospital may not survive if services are downgraded or relocated.
They said they would not stand "idly by and see lives put at risk" by proposals to close the hospital's A&E department.
Hywel Dda Health Board says no decisions about changes have been made.
In December, the board unveiled proposals to change the provision of emergency treatment at its hospitals in mid and west Wales.
In each option, A&E at Llanelli would be downgraded to what the board calls an "urgent care centre," with full A&E services available at Glangwili in Carmarthen or Withybush in Haverfordwest.
The board said it was embarked on a "listening and engagement exercise" to formulate options which would go to formal consultation.
"The listening and engagement exercise aims to increase understanding of the challenges facing the NHS in Wales," said the board.
"All staff, stakeholder groups and the public are free to express their views and opinions to shape the options that will eventually go forward for formal consultation later in the year.
'Safety and quality'
"Our services must meet safety and quality standards and we would not consider unsafe solutions.
"Absolutely no decisions have yet been made, but we are clear that no change is not an option."
The board added that a series of events inviting public views about the proposals would continue at various locations throughout February.
Llanelli AM Keith Davies has claimed the ambulance service is already bypassing Prince Philip Hospital, taking emergency patients to Carmarthen or Swansea instead.
The Wales Ambulance Service responded by saying it worked to ensure patients were sent to the "appropriate healthcare setting".
Last week senior medical staff at Bronglais Hospital, Aberystwyth, signed a letter saying they had lost confidence in Hywel Dda.
'Listen and engage'
They said they did not believe the board was committed to delivering services locally and that its plans were contrary to those agreed with medical staff.
On Friday, hours before a public meeting to discuss the future of Bronglais, Health Minister Lesley Griffiths intervened, saying she expected officials to "listen and engage" with clinicians and the community.
More than 500 people attended the meeting in Aberystwyth, many concerned that services could be moved to Carmarthen.
At the meeting members of the public, hospital staff and politicians supported a motion to reject the health board's plans for Bronglais.
The motion also called on Mrs Griffiths to authorise the preparation of a new plan for "a health service for mid Wales in mid Wales."