A poster pinned on doors of a tower block telling residents to only speak English is being treated as a "racially aggravated public order incident" by police.
The signs, carrying the title "Happy Brexit Day", were found stuck on fire doors across all floors of Winchester Tower in Norwich on Friday morning.
It told residents of the flats that "we do not tolerate" people speaking languages other than English in the building.
Norfolk Police confirmed they were investigating the incident after officers visited the block on Saturday night after all the posters had been taken down.
In a statement, the force said: "Those posters kept by residents have since been seized for forensic inquiries and we will be working with the council to examine any available CCTV.
"There is no place in society for hatred and intolerance. Nobody should have to face intimidation because of who they are and it is more important than ever that we stand together in the face of hostility.
"We remain committed to helping people feel safe and secure as they go about their lives.
"The matter is being dealt with as a racially aggravated public order incident and anyone with information which could help officers with enquiries should contact Norfolk Police on 101 quoting crime reference 36/7964/20."
The posters, images of which were shared on social media, were first spotted on the day the UK officially left the EU and said: "We finally have our great country back".
Police said no arrests had been made.
It added: "We do not tolerate people speaking other languages than English in the flats.
"We are now our own country again and the the Queens (sic) English is the spoken tongue here.
"If you do want to speak whatever is the mother tongue of the country you came from then we suggest you return to that place and return your flat to the council."
Writing on Twitter on Saturday, Norwich City Council said: “You may have seen a photo of a poster that has appeared in one of our properties.
“Norwich has a proud history of being a welcoming city, and we will not tolerate this behaviour.
“As soon as we became aware of this incident, we reported it to @NorfolkPolice and they are investigating.
“We take this very seriously and encourage residents to contact us or the police if they have any concerns.”
According to the BBC, which spoke to a resident, the signs were left on fire doors across all 15 floors of the block of flats before being removed by the caretaker.
Mike Stonard, a cabinet member on Norwich City Council, told the Eastern Daily Press: “I absolutely condemn this abhorrent poster. Whoever put it there has committed a hate crime, it is as simple as that.
“Many people voted for Brexit for a range of different reasons, however I am sure not many of them will condone this kind of thing.”