Dyfed-Powys Police said a 22-year-old was taken to hospital after "a serious assault" in Dillwyn Street, Llanelli, on Friday.
The attack happened between 19:45 - 20:15 GMT.
Two men, aged 30 and 52, have been arrested and were in custody on Saturday afternoon.
A man has been taken to hospital with serious injuries following an assault in Carmarthenshire.