More than 5,000 people have signed a petition in support of a college lecturer who was sacked for defacing a poster about the counter-extremism programme Prevent.

David Muritu, a maths lecturer, wrote the words “racist” and “should be illegal” on a poster promoting the Prevent scheme at Sandwell college in the West Midlands.

Muritu, who is also a branch secretary for the University and College Union (UCU), described the college’s decision to sack him for gross misconduct as “completely disproportionate”.

Speaking before a protest by supporters outside the college on Friday, the lecturer said the college had “taken the opportunity to get rid of me on the flimsiest of excuses” because of his union role.

He said: “I have apologised for what I did. It was a rash act. I don’t think it’s appropriate that I go around scrawling across posters but I think the reaction is completely disproportionate. I really hoped that they would accept my apology.”

The UCU plans to formally appeal against the dismissal next week and has said it will ballot for strike action if the college’s response is “not positive”. A petition launched by the union earlier this week had gained more than 5,100 signatories by Friday morning.

Muritu, who has worked at the college for six years, said he believed he was the subject of “clear victimisation” for his leading union role in industrial action at the college in recent years. Last year, members of the UCU went on strike for five days at the college before securing an improved pay deal.

The lecturer said he had been accused of gross misconduct on “trumped up charges” last year following a disagreement with a colleague about strike action. He said the gross misconduct claim had been withdrawn following a challenge by his UCU branch.

“For several years they have been trying to victimise me while we’ve been having all this success,” he said.

“I put up with what they do to me and how they treat me so I can do the enjoyable parts of my job. I could go straight back there and I feel it wouldn’t be much different. I love my job.”

Muritu said he had previously raised concerns about the Prevent programme using more conventional routes but defaced the poster in “a moment of frustration”.

The government scheme places a legal duty on schools and colleges to raise the alarm about students they think could be at risk of being radicalised. But educators have raised concerns about its effectiveness and fear it is undermining efforts to build an inclusive environment in schools and colleges.

Sandwell college did not comment specifically on Muritu’s allegations but said it was “dismayed” by his recent remarks in the media. A college spokeswoman said: “As with all colleges, Sandwell college takes its government statutory safeguarding and prevent duties very seriously and has the same high expectation of its staff. We pride ourselves on promoting our core values of respect, tolerance and promoting diversity in a positive manner.

“The college is dismayed with recent comments made by Mr Muritu reported in the media. However, it does not comment in detail on internal procedural matters. The college has and continues to support the contribution of all recognised unions.”