The Welsh assembly's presiding officer has called some campaigners for the National Eisteddfod's Welsh-only rule "language fascists".
Dafydd Elis-Thomas is a former president of Plaid Cymru
Lord Elis-Thomas' comment was made in a public meeting after some members of the audience booed a speaker who addressed the meeting in English.
Laura McAllister was greeted with cries of "rheol Gymraeg" (Welsh-only rule) at the Tomorrow's Wales meeting.
Lord Elis-Thomas made the comment in Welsh "ffasgwyr iaith" (language fascists) when they refused to allow her to continue.
Professor McAllister had begun talking in Welsh at a discussion on changes to assembly powers, but switched to English because she said she felt more comfortable.
The former Plaid Cymru election candidate was the party's appointee on the Richard Commission, which was set up to investigate the workings of the assembly.
Welsh is her second language.
About a dozen people walked out of the meeting in protest at English being used, but many more had joined in the jeering.
One of those who left the meeting was historian and broadcaster Hywel Teifi Edwards.
He said: "The Welsh-only rule exists in the eisteddfod.
"I came to this meeting expecting that rule to be respected.
"And I happened to be sitting behind Dafydd Elis-Thomas, of all people, who was calling those who were calling for Welsh to be spoken, fascists.
"One starts to wonder where we're heading in Wales."
After the meeting Prof McAllister, whose second language is Welsh, said she apologised if she had caused any offence, which was not her intention.
She said she had told the organisers of the meeting her intention to speak English and was under the impression that it was acceptable.
The event was organised by Tomorrow's Wales, the movement chaired by Archbishop of Wales Barry Morgan to support the implementation of the Richard Commission, which called for the assembly to have full law-making powers.
Sion Brynach, who organised the event of behalf of Tomorrow's Wales, apologised.
He said the organisers regretted what happened and that it had overshadowed a very important discussion.