Plaid Cymru have beaten Labour to take one of the seats in Cardiff's Ely ward following a by-election.
Andrea Gibson received 831 votes in what is normally a strong Labour constituency.
Labour candidate Irene Humphreys came second with 779 votes.
The Labour leader of Cardiff council described it as a "bad day" and time was needed to "reflect, learn and rebuild."
Following her win grandmother-of-three Councillor Gibson said people in the ward "had courage to do something different".
Cllr Gibson, who has three daughters and spent 25 years as a mental health carer, said: "I've always seen myself as an advocate for people who have not got a voice..
"I've always been interested in helping people stand up for themselves."
She added: "Ely people are just brilliant. There's a fantastic community there. The people are very close-knit.
"Ely does get a bad rep but it's not deserved. There's some brilliant people in Ely.
"I would like to thank the people of Ely for having the courage to make a change and do something different."
The by-election was called after the death of Labour's Jim Murphy, who died on December 1.
The 72-year-old, who worked at Allied Steel and Wire before becoming a Labour councillor, was elected in 2012 to represent the Ely ward.
The other two Ely seats are held by Labour councillors Russell Goodway and Susan Goddard.
Plaid leader Adam Price was one of the first to congratulate Cllr Gibson on her win.
Neil McEvoy AM, who helped run the campaign, described the win as a "political earthquake".
"This is a political earthquake in one of Labour's strongest wards in Wales. The neglect of a proud, working class community caught up with Labour last night," he said.
Cardiff council leader Huw Thomas congratulated Andrea Gibson but described it as a "bad day".
He said: "Many congratulations to Andrea Gibson and Plaid Cymru on their win in Ely, and heartfelt commiserations to Irene Humphrys. Today was a bad day, in a week of absolute political chaos nationally.
"We need to reflect, learn, and rebuild."
