A “Brexit day” poster that demanded foreigners in a Norwich tower block speak English or leave the UK has prompted a furious backlash among residents who staged an anti-racism demonstration and declared their block welcome to everyone.

Norfolk police are investigating a flyer stuck to doors on all 15 floors of Winchester Tower on Friday as a racially aggravated incident after it said non-English speakers had “infected this once great island”.

Toni Maguire, a fourth-floor resident who is in her 70s, said she and her neighbours were appalled by the message, which was printed under the title “Happy Brexit Day”.

On Sunday, while taking part in an anti-racism protest outside the block, she said: “We all thought it was absolutely disgusting. There is a fire door on every floor and there was a poster on each one. It was outrageous.”

After the offending posters were removed as forensic evidence by police, residents posted their own posters, saying “Everybody is welcome in Norwich” above a heart symbol.

The racist flyer appeared on the doors on Friday morning, the day the UK formally left the EU.

Addressing residents, it said the “Queens [sic] English is the spoken tongue here” and hailed the moment “we finally get our great country back”.

It said those unwilling to speak English should leave the UK. The text read: “You won’t have long till our government will implement rules that will put British first. So, best evolve or leave. God Save the Queen, her government and all true patriots.”

Maguire said: “I’m half-Irish. I don’t think it was directed at me because I’ve got an English accent. It was directed at eastern Europeans from the block. I’ve heard people muttering things against them in pubs, ‘over here taking our jobs’, that sort of nonsense. They have been taking the jobs that the British don’t want to do.

“When I was rushed into hospital last year, the nurse and doctor who saw me were eastern European and they could not have been better.”

Hugh Stanners, from the Norwich branch of Stand up to Racism, who organised the demonstration, said: “It is a really nasty racist poster, obvious from someone who is far right or fascist, because it talks about ‘infecting our once great island’. It appears to be the far right are gaining confidence, or seeing an opportunity, from us leaving the EU. People are very upset about it. It wouldn’t be right to portray it as a lone nutter. I think they are part of an organisation.”

Adrian Żółkowski-O’dell, the chair of the Norfolk Polish Heritage Group, said the flyer was “appalling and shameful”. He pointed out that up 9,000 Poles were estimated to be living in Norfolk.

Żółkowski-O’dell, whose father was a Polish airman who fought in the second world war before moving to Norfolk, said: “Norwich is not known for its racism particularly, but I do know that since the result of the referendum Poles have felt uncomfortable. They were shocked by the result.

“We’ve interviewed recent arrivals who have felt there’s been a change in attitude towards them, but nothing tangible until this. This is the worst incident so far.”

Tom Haczewski, a Norwich-based web designer, thought his Polish grandfather would have been “horrified” by the posters.

He said: “I know Norwich to be one of the most tolerant and liberal cities in the UK – we’re proud of our heritage and accept everyone that comes here. I know that this poster is a one-off and we would be ashamed to see this become any more frequent in our city.”