Chants of "Gwynfor, Gwynfor" rang out at the Carmarthen East and Dinefwr count after Plaid Cymru's Adam Price more than doubled his majority.
Adam Price said Labour had "thrown everything at him"
Supporters were paying tribute to the late Plaid Cymru MP Gwynfor Evans, who died last month, and who memorably won the seat for the party in the 1960s.
Mr Price has been leading a campaign at Westminster to impeach Tony Blair over the Iraq war.
He said Labour had thrown everything it could at him in the past four weeks
His 6718 majority bucked the trend of Plaid Cymru failing to make gains in the rest of Wales. "They threw everything at us in this campaign. They made us the only target seat in the UK and subjected me to personal attacks".
Nia Griffith said Llanelli's 'chapel culture' helped her win
Mr Price, who won the seat from Labour's Dr Alan Williams in 2001, has been one of the MPs leading a nationalist campaign, claiming Tony Blair had "misled" Parliament over the Iraq war.
"That's what you get when you question this prime minister and they still lost. If the charge against me was that I was a persistent critic of this right wing government then I'm guilty."
Labour candidate Ross Hendry saw his party's votes drop by almost 3,000 on the 2001 election despite an improved turnout of 72.2%.
Labour held Llanelli with a majority of 7234 despite former government minister Denzil Davies retiring after more than 30 years and the imposition of the controversial all-women shortlist.
Plaid Cymru who came within a handful of votes to taking the seat two years ago in the assembly elections were clearly disappointed with the result.
Winning candidate Nia Griffith said her win was because of Llanelli's "socialist and chapel culture".
She admitted there had been concerns about Iraq and other unpopular national issues when canvassing.
But she said: "We got our act together locally. People said they would vote for us with 'buts'. Now we must take notice of these issues."
Plaid's Neil Baker said he was confident the party would do well in the next assembly elections.
He said the message he was getting was people would vote for Plaid in local and assembly elections but were turning to the Liberal Democrats when casting their parliamentary votes.