A student speaker who was meant to be arguing for the right to offend dramatically turned on their own side to attack Kathleen Stock, as part of a tense debate at the Cambridge Union last night.

Kass Kaldicott from Trinity College, pivoted from arguing for the right to offend to attacking Stock, saying: “I do not believe trans rights are up for debate and I believe the Union was wrong to invite Dr Stock” – this was met by a round of applause in the chamber, as well as several interventions from Stock’s defenders. One floor speech condemned it as an “unsolicited and unhelpful attack”.

Stock, who at several points sounded upset in her speech, said she was not going to address what was said about her by previous speakers. Nor did she mention trans rights in her speech.

President Lara Brown urged speakers to engage with the motion as opposed to criticising particular speakers.

After the debate, she told Varsity: “The first proposition speaker auditioned for their slot. While the majority of their speech did not resemble the audition, as a free speech society, the Union will always allow speakers their full time to make their case.”

The Union is no stranger to heated moments, but the atmosphere in the chamber during last night’s debate reached a new level of tension.

Speaking in favour of the motion was philosophy professor Kathleen Stock, an academic who claims she was forced out of Sussex University for her views on transgender rights last November.

Stock argues that while transgender people are entitled to respect for their choices, biological sex is an immutable fact and ignoring this might threaten the rights of women to safety and security.

Activists gathered to protest the event, with one of the groups, Trans Liberation Cambridge, writing on Instagram: “Protesters are opposed to Dr Stock’s comments on transgender people, which include calling trans women the ‘dress-wearing equivalents’ of ‘incels’, referring to gender transition as ‘self-mutilation’, and stating that gender transition is motivated by predatory sexual desire”.

Among the concerns of the group was the fact the debate took place during Trans Awareness Week and three days before Transgender Day of Remembrance.

Protesters argued outside the Union’s back entrance and chanted: “No Terfs on our turf”, “What do we want? Trans rights. When do we want them? Now!”, and “Shove your transphobia up your ass”. Throughout the two debates there was the sound of drums, chanting, clapping and cheering.

The benches of the chamber were packed, with several attendees forced to stand at the entrance. Interventions in speeches (points of information) were common and many put themselves forward to make smaller speeches (floor speeches) in between the main speakers.

The debate was filmed as part of a documentary for Channel 4 exploring the impact of the current cultural climate around gender issues – cameras flanked the central chair.

During the debate, Lara Brown twice mentioned that the Union’s welfare officer was available if either of the debates offended attendees. This seems to have been a response to internal concerns within the Union about the event’s impact on members’ wellbeing.

However, Brown vetoed further linking the welfare events to the debate, writing on the Union’s publicity group chat before the debate began: “suggesting that this debate is any harmful would undermine the purpose of the society and the reasons we’re hosting this debate”.

She added: “I’m happy to announce Alex [Horan]’s weekly welfare hours but not in a way that’s remotely linked to this debate”

There were fewer protesters outside the Union than gathered outside Gonville & Caius to protest Helen Joyce. Joyce had been invited to the College by Professor Arif Ahmed, who also spoke at last night’s debate.

Stock said on Twitter after the debate: “Thanks to all who were positive about my speech in Cambridge debate tonight.

“Can’t say I had the most fun evening but I respect free speech of protestors and those defaming me in the chamber. Interesting points made on both sides. Also, we won.”

Lara Brown told Varsity: “The Cambridge Union Society was established over 200 years ago to facilitate the free exchange of ideas. Last night’s debate was an example of this core purpose in action. The debate was, by its nature, adversarial.

“It featured different views, whilst also encouraging participants and members to consider and adapt their views throughout. Members were given the opportunity to comment on the content of the speech, and many chose to respond thoughtfully and constructively. The Union is proud to have hosted such a thought provoking discussion.”