Hywel Dda University Health Board announced reductions in paediatric and maternity services at Withybush Hospital in Haverfordwest, in 2014.
Save Withybush Action Team (Swat) is calling for an urgent review of the changes.
The group marched from the town's cenotaph to the hospital on Sunday.
Hundreds of people campaigning against changes to services at a Pembrokeshire hospital have staged a protest march.