Prime Minister David Cameron's spokeswoman on Monday said the government "will not tolerate intolerance" after a series of racist incidents were reported following Britain's decision to leave the European Union.
"We should be absolutely clear that this government will not tolerate intolerance... intimidating migrants, telling them they need to go home," his spokeswoman told journalists.
The Polish Embassy in London earlier said it was "shocked and deeply concerned" by incidents of abuse directed at the Polish community.
They include the posting of laminated cards reading "Leave the EU - no more Polish vermin" to members of the Polish community in Huntingdon, near Cambridge, on Saturday and reports of racist graffiti scrawled on a Polish community centre in Hammersmith, London.
"We would like to thank for all the messages of support and solidarity with the Polish community expressed by the British public," the embassy said.
London mayor Sadiq Khan on Monday placed the city's police force on alert following the incidents.
Khan said he took "seriously my responsibility to defend London's fantastic mix of diversity and tolerance.
"I've asked our police to be extra vigilant for any rise in cases of hate crime, and I'm calling on all Londoners to pull together and rally behind this great city.
"It's also crucial that we don't demonise the 1.5 million people in London who voted for Brexit, we must respect their decision and work together now to get the best deal for London."
Cambridgeshire Constabulary's Nick Percival said the force was looking into the posting of racist cards.
"We are... taking it seriously as it does represent a hate crime. We would encourage anyone who is either a victim or is aware of the source of this to come forward."
Other incidents were also reported on social media.
John O'Connell, from anti-racism group Far Right Watch, said they had recorded over 90 incidents in the last three days, ranging from "verbal abuse up to physical violence."
Sky News journalist Adam Boulton tweeted that he had seen three "'when are you going home?' Racist incidents aimed at EU citizens."
Finance minister George Osborne called for "unity of spirit and purpose", and urged Britons to "condemn hatred and division wherever we see it."
"Britain is an open and tolerant country and I will fight with everything I have to keep it so," he said.
Conservative politician Sayeeda Warsi, who made a high profile switch to the Remain camp, blamed the highly-charged campaign for enflaming tensions.
"Long after the political bus moves on we leave problems on our street," she told Sky News.
"I've spent most of the weekend talking to organisations, individuals and activists... who monitor hate crime, and they have shown some really disturbing early results from people being stopped in the street and saying look, we voted Leave, it's time for you to leave.
"And they are saying this to individuals and families who have been here for three, four, five generations. The atmosphere on the street is not good."