Keir Starmer's Labour party has lost the Caerphilly seat to Plaid Cymru for the first time in a century.
The defeat deals a blow to Starmer's leadership ahead of the Welsh elections in May.
"We want a better deal for every corner of Wales,"
"The big parties need to sit up and take notice. Wales, we are at the dawn of new leadership; we are at the dawn of a new beginning."
Lindsay Whittle, the winning candidate, described Labour as a "dying beast" that is on its way out after 100 years of power in Wales.
Whittle secured a 3,848-vote majority in the Welsh Senedd assembly seat.
Author's summary: Labour loses Caerphilly seat to Plaid Cymru.