Students at Cardiff University have expressed anger after anti-abortion activists returned on the first day of the new term to protest outside the students’ union.

It was the fourth demonstration to be held at the university in recent weeks and again featured large-scale graphic images, which students say cause “undue distress and trauma”.

An external anti-abortion organisation called the Centre for Bioethical Reform UK (CBR UK), which recently targeted the pro-choice Labour MP Stella Creasy, is behind the series of protests.

On Monday, members posted a photo of the latest Cardiff demonstration on Facebook with the message: “Back outside Cardiff uni for the first day of term as we continue our educational displays for the students. Here until 14.00.”

Cardiff students have found themselves the focus of anti-abortion demonstrations after the students’ union voted last November to adopt an official pro-choice stance. Students have set up a pro-choice society.

Though the UK student body is overwhelmingly pro-choice, in recent months the anti-abortion lobby has become more visible on campuses as supporters try to capitalise on an ongoing debate about freedom of speech in universities.

There are now an estimated 14 anti-abortion societies in university student unions around the UK, including at Cardiff. Some student unions have been threatened with legal action if they try to prevent anti-abortion groups from opening on campus.

Jackie Yip, the president of Cardiff University students’ union, said it had received complaints about Monday’s protest from staff, students and members of the local community, who said the displays were causing “significant distress and upset”. She was concerned too that the graphic imagery was posted near a nursery, in clear view of children.

“The welfare of our staff and students is a priority, that is why we are seeking support in preventing the continuation of these displays,” she said. “This is not about suppressing freedom of speech. This is about calling out harassing behaviour and showing compassion to those who feel affected by these actions.”

In an earlier Facebook post, Yip said: “We are speaking to the local council, the university and the police to build a case that these actions are causing undue distress and trauma to our students.”

A spokesperson for Cardiff University said it encouraged freedom of speech, robust academic debate and the right to peaceful and lawful protest, but warned that protesters would be reported to police if there was any evidence of law-breaking.

“We recognise the strength of feeling and the university is monitoring the situation and continues to work alongside our students’ union and other partners to do everything we can to address these concerns,” the spokesperson said.

There is concern that a visit to Cardiff next month by Dr Pam Lowe, a senior lecturer in sociology and policy at Aston University who researches anti-abortion activism across the UK, could be targeted by protesters.

She recently told the Guardian: “The anti-abortion movement has collectively recognised they have got an ageing population. They are setting out strategically to try to engage more younger people and get younger people advocating against abortion.

“This is a deliberate policy, that’s quite new. It’s an orchestrated campaign, but where exactly it originates is unclear to me. “There’s no sign that it’s really being successful at the moment.”

CBR UK denied any misconduct and said the students were part of “a generation who have sadly been convinced into thinking that ideas are violence and universities a place where everyone must hold the same view”.