A poster showing anti-Semitic messages were spotted at a lockdown protest in Nottingham.
One sign was photographed at the scene reading "Zionist rats control the media".
Police believe the incident happened on December 13 and was later reported to the force.
Ben Cooper, a freelance journalist at the scene, told Nottinghamshire Live: “It was really shocking to see this sort of offensive language being used so openly at a protest, which was riddled with slogans and dangerously misleading information being spread by the organisers.
"Jews have been targets for appalling hostility and abuse like this for centuries. The reemergence of antisemitism in the 21st century, at such a tense, polarised time in history, is a very serious and worrying phenomenon which should be treated extremely seriously.”
Nottingham City Council's deputy leader, councillor Sally Longford, said on Twitter she reported the posters to the force who are now investigating.
Nottinghamshire Police's Inspector Paul Gummer said: "We are working to identify the individual or individuals who displayed these posters and our enquiries are ongoing.
“Nottinghamshire Police takes all reports of hate incidents very seriously and works closely with partner agencies and other forces to ensure that any allegations of racism are investigated thoroughly.
"Hate crime in any guise will not be tolerated whether in person or on social media.
“Anyone who has any information about this incident is asked to call Nottinghamshire Police on 101”
