Police detained Friday four people for an anti-Serb banner unfurled by Croatian football fans that sparked outrage in the country and was condemned by officials.

A photo of about a dozen Dinamo Zagreb fans standing on an improvised stage with a giant banner that read "We will fuck Serb women and children!" was put on Twitter by the Serb National Council (SNV), an umbrella organisation for Croatian Serbs.

The photo was taken late Thursday in Zagreb by a Croatian photographer who put it on Facebook and said the fans chanted 'Kill, kill a Serb!'.

The fans, holding torches and dressed in black T-shirts, also waved 1990s paramilitary flags with a pro-Nazi slogan from Croatia's World War II regime.

Hardcore Dinamo Zagreb fans are known for their violent behaviour and often chant anti-Serb slogans during matches.

"It is scary and disturbing for everyone, not just Serbs," SNV head Boris Milosevic told N1 television.

"It is sad to see that young people in Zagreb are so full of hatred that they write such disgusting messages shouting 'Kill, kill a Serb'," he said.

Police said in a statement that they had detained four people in connection with the incident.

Conservative Prime Minister Andrej Plenkovic said he had not seen the photo but "most strongly condemned" it.

"For me there is no dilemma, it is unacceptable."

Ethnic Serbs are Croatia's largest minority, accounting for around four percent of the population of 4.2 million.

Respect for their rights was a key criteria for Croatia's 2013 entry into the European Union.

But ethnic Serbs warn of growing violent acts, hate speech and other forms of discrimination against them.

Relations with the ethnic minority remain fragile since Croatia's 1990s war with Belgrade-backed rebel Serbs who opposed the country's independence.