UKIP has accused Plaid Cymru leader Leanne Wood of "tacky and baseless" jibes after calling Nigel Farage the "voice of the far right".
Ms Wood made the reference during First Minister's Questions while discussing Carwyn Jones's debate with the UKIP leader on EU membership.
A UKIP spokesman said she had acted "crudely and immaturely".
In response, Plaid Cymru said UKIP's approach was "based on division and the scapegoating of vulnerable people".
Ms Wood said: "First minister, last night you participated in the first major public debate on the future for Wales within the European Union.
"You went head-to-head with the voice of the far right.
"Do you think your performance helped or hindered the Welsh campaign on the EU?"
The UKIP spokesman said: "She so crudely and immaturely labels the one politician fighting for British sovereignty, against unelected foreign control, in order to put the interests of the men and women of the UK first, as 'far right'.
"Perhaps she should reflect on her own 0.6% share of the vote in the general election, compared to UKIP's support a massive 21 times higher, making UKIP the third most popular party in the UK, before she uses tacky and baseless labels to accuse a party backed by 4m voters as representing extreme or niche politics."
Responding to UKIP's criticism, a Plaid Cymru spokeswoman said: "UKIP's political narrative is based on division and the scapegoating of vulnerable people.
"Plaid Cymru is the party for everyone in Wales, and the only party that is putting forward the positive alternative that Wales needs in health, economy and education."
During their exchange in First Minister's Questions on Tuesday, Ms Wood told Mr Jones that "a new Europe is possible, but you failed to put that positive vision in that debate last night".
Mr Jones said Ms Wood "fails to outline how she would have done anything differently, what message she would put forward".
Liberal Democrat AM Eluned Parrott criticised Ms Wood's tactics, saying on Twitter: "If Plaid want Wales to remain in Europe, it's not wise for their leader to stand here and drive wedges between people on the same side."