The UK is again in mourning. Following on from the attack on Westminster Bridge and the Manchester Arena suicide bomb, yet another terror attack has stolen the lives of seven people and injured many more.

Despite jaunty social media posts about the spirit in which Londoners were 'keeping calm and carrying on', this is a country in shock. Not since mid 2005, when plots were foiled in the aftermath of the 7/7 bombing has the country felt like this.

At a time like this, it's tempting to focus our rage on those saying or doing inappropriate things rather than the terrorists themselves, so let's start by saying that the only people responsible here are the men who carried out this despicable crime. Anyone found to have helped them must be found, put on trial and sent to prison for a very long time indeed.

Yet the wake of an attack like this does tell us a lot about our society and our friends. In particular, the President of the United States has, once again, courted controversy with his tweets.

'At least 7 dead and 48 wounded in terror attack and Mayor of London says there is 'no reason to be alarmed!'' tweeted Donald J Trump. He had chosen to take the Mayor's words out of context. What Sadiq Khan had actually said was, 'Londoners will see an increased police presence today and over the course of the next few days. There's no reason to be alarmed.' He was clearly talking about there being no need for alarm at the presence of more armed police.

Khan has been the victim of this kind of misrepresentation before. He was accused by various hostile political outlets of claiming that terrorism is part and parcel of living in a big city like London. He actually didn't say that. He said, 'Part and parcel of living in a great global city is you've got to be prepared for these things, you've got to be vigilant, you've got to support the police doing an incredibly hard job.'

Who was leading the misrepresentation on that occasion? Donald Trump Jnr. Fake news websites and conspiracy theorists leapt on Khan's quote and used it as a chance to attack him. Khan, as he has been since he was elected, seems to be guilty of the crime of being Muslim while Mayor – an unforgivable thing in the minds of the alt-right which is desperate to exploit tragedy and grief for its own ends.

Trump's tweets were in marked contrast to the solidarity and compassion shown by other worlds leaders. Monuments worldwide were lit up in the colour of the Union Flag. Of course that didn't change anything, and lighting a candle will never defeat terrorism, but that isn't the point. While the investigation is ongoing, world leaders can't do anything to make the situation better – but they can make it worse. That's what Trump did this weekend.

He's drawing on a giant festering cesspit, exemplified by the likes of Katie Hopkins who rush headlong into comments without knowing facts – or even caring about facts. They use tragedy to boost their own profile and push their predetermined agenda.

'The unarmed British police ran away from Muslim attackers, leaving folks defenseless. 2nd Amendment, anyone?' wrote Bryan Fischer, a radio host for American Family Radio. This was based on a single eyewitness testimony from a live blog. At the time Fischer's tweet was posted, accounts were already coming out of the astonishing bravery of off-duty and unarmed British police rugby tackling the perpetrators and being seriously injured in the process.

But these facts are of little concern to those who want to make political capital from a tragedy. People in London are dying, injured and bereaved and Fischer wanted to have an argument about the second amendment. We could do without this rubber-necking ghoul in the UK. The other fake news outlets which are pushing the myth of British police running away should also be boycotted. Naming them here would simply give them more clicks, so I won't do it.

Cool heads are required. Our response must be tough and thorough. We must name this as what it is – a despicable act of terrorism. We must also as a country do all that we can to root out salafi-jihadism from our streets. This means action from both within and without the Muslim communities of our great cities.

This effort will not be helped by the spreading of fake news about our brave police and the deliberate misquoting of our Mayor by outsiders. Our American cousins should know this – the UK withstood the Blitz and the IRA and we will withstand the current wave of terror attacks. You can either help us out or you can spread fake news about us. Make the right choice.