There is a "culture of institutional racism" at Bristol City Council, black and ethnic workers have claimed.
An investigation by the Bristol Post revealed complaints of bullying and discrimination, which victims said were not dealt with effectively.
The authority admitted equality and diversity had "not been anywhere near where it should be" historically.
It said a "spike" in complaints was due to greater awareness of issues among minority groups.
The council said it was committed to cultural change and a review was under way.
The Bristol Post reported the authority was "harbouring a shocking, deep-rooted culture of institutional racism".
The investigation found a "catalogue of casual racism", including black people being told "the room doesn't need cleaning" when they entered meeting rooms, and senior black people told to clean telephones.
It also uncovered several cases in which witnesses said they were punished for speaking out.
A council spokesman said staff-led groups representing minority groups, including black and minority ethnic (BAME), were relaunched last August "as part of a commitment to equality and inclusion".
Chair of the council's BAME staff-led group, Saida Bello, said staff were "invited to come forward with any concerns they had".
"By taking this pro-active approach it undoubtedly resulted in greater awareness and therefore a spike in complaints."
Since December, the council has interviewed over half the complainants, she added.
Council executive director Mike Jackson said: "Historically equality and diversity hasn't been anywhere near where it should be at the council.
"This means, despite recent strides to create an inclusive organisation, attitudes and cultures take longer than desired to change."
He added a review of HR policies, including a "systematic review for the presence of unconscious bias", was under way.
"[We] believe the work we are doing will provide long-lasting cultural change and ensure all colleagues are treated fairly."