But I’m not racist," protested one of my oldest British friends. “Just because I’ve voted for Brexit doesn’t mean I’m racist." Indeed, it is hard to think of a more multicultural man, a Londoner and an internationalist to the core. His problem is that he’s white, which, in the eyes of some of his critics, put him in the company of racists who backed the Brexit campaign for Britain to exit the European Union.

His critics were wrong and my friend shouldn’t have had to answer any charges, but that’s what the Brexit referendum has done—it’s divided us. Although my friends have made up now over a night of drinks in north London pubs (thousands of miles away, they kept me updated), this referendum has exposed the darker side of politics in Britain.

After months of campaigns vilifying foreigners, there’s been an explosion of racist incidents across the UK, including—sadly—in London. Across Britain, people of African origin have been called the ‘N’ word, Asians abused as Pakis, leaflets (carefully laminated in case it rained) dropped outside Polish homes calling them “vermin", all advising non-Britons to leave. It’s as if the nation of 65 million had somehow turned the clock back on itself—to an ugly time that had been erased from its memory.

For decades now, Britain has lived in the illusion that it has “sorted racism", lulled by the air of Cool Britannia in the Tony Blair years, despite repeated warnings by anti-racist activists not to allow complacency to set in.

Any visitor to London, in particular, can be forgiven for believing racism is a settled issue in the British capital, that no one is racist in the city, that the colour of your skin or passport doesn’t matter—not here, not now. There is no doubt that London has probably made greater strides against racism than any other city on this planet, but at one level it is also a convenient and dangerous myth that chimed in when Britain was in a phase of economic growth, for which it needed skilled immigrants.

In the process we have all—the city, my friends, admiring visitors, leaders—allowed ourselves to become complacent.

This is why this latest explosion in racist acts has taken everybody by shock, where the need was to have been vigilant after the ugly Brexit campaign.

David Isaac, Chair of the Equality and Human Rights Commission, a body created by Parliament and charged with building a fairer society, said: “Britain has a strong and proud history of valuing diversity and challenging hatred and intolerance…

“The referendum vote showed that the overwhelming majority want to engage in the democratic process, and this is heartening. But the campaign and the result have resulted in divisions in our society which will take efforts to heal. We and our political leaders now need to pull together in the national interest to heal those divisions and unite the country against racism, hatred and intolerance."

How to heal though? After all, Prime Minister David Cameron himself referred to a “swarm of people" in pre-referendum comments about refugees. Fortunately, there are lessons in Britain’s history that it can revisit and invoke in order to educate children and spread awareness that racial (or religious or homophobic) hatred is a crime in the UK that can and will lead to punishment.

Who takes the lead here? Given the groundswell of public revulsion, I believe it is the progressive civil society—occupying the wide swathe of civility between the family and state—that must take the lead. Britain has been here before and overcome the challenge. Racists tried to stand in the way of Gujarati refugees from Uganda, Punjabi immigrants from India and Caribbean people.

Both Asians and the Caribbean people were able to challenge, confront and overturn this racism, with strong support from trade unions and the Labour party, the main reason most ethnic minorities continue to support the party of the Left in Britain.

Every wave of immigration and every phase of economic decline in the UK has led to a spurt in racism. But the previous spurt (directed at a particular minority) does not simply disappear with every subsequent wave of immigration or phase of low economic growth—it merely subsides. The overall effect, therefore, is one of layers of racism, some of it at low level, some tucked away outside the cities, and some overt and full-on as we have seen most recently after the Brexit results.

The 1970s and 1980s saw both Blacks and Asians stand up to racist violence and stamp it out in parts of London. One of the most famous acts of resistance took place in Southall—the best known of all of the Little Indias spread across the world and the original home of the Punjabi diaspora in the UK.

It began with the killing of a teenager, Gurdip Singh Chaggar, in April 1979, an event that had been greeted by National Front leader John Kingsley Read with the grizzly words: “One down, a million to go." Street demonstrations followed, marked by violent clashes between Punjabi youths and White racists on the one hand, and Punjabis and the police on the other. In one such incident, a young anti-racist teacher from New Zealand, Blair Peach, was killed.

The anger on the streets of Southall boiled over dramatically two years later, when an attack by racist skinheads on an Asian woman led to a large group of young Indians laying siege to a pub where skinheads were attending a rock gig. The pub was firebombed and burnt down.

The last act: in 1987, Labour councillor Ranjit Dheer ordered the closure of a bar in a housing estate with 3,000 almost entirely White-occupied houses and flats. Skinheads frequented the bar and spilled out at nights to target Asians for drunken abuse and violence.

“We gave them three months to clean up," Dheer told me. “They resisted and were very arrogant. The idea that Asians could be in control was anathema to them. They threatened us.

“We disbanded the residents’ association and one night, sent in bulldozers and flattened the bar. In its place, we built a health centre and a drop-in centre for women and the elderly. That night in 1987 was a test of our political will."

The bulldozers have long been put away. There’s no need for them now. But after Brexit, it is time fair-minded Britons of all skin colours regrouped and reminded themselves of the battles they have won against racism and invoke the spirit of Southall.

Dipankar’s Twitter handle is @Ddesarkar1