There was a great commotion in central London last night. A police helicopter hovered over The Spectator's office making a din, police sirens sounded and thudding music rattled the windows. I found out why when I left the office and walked via Parliament Square to Whitehall.

There was an anti-Trump protest outside Parliament – #stoptrump was the theme – coinciding with the (non-binding and pointless) debate inside Westminster Hall, about President Trump's state visit to the UK later this year. The protest was a very slick affair. There was a massive TV screen broadcasting anti-Trump videos, and speeches blared out over a speaker system. But there was just one thing missing: a crowd to match the scale of the event.

In the square itself, there were hundreds of people – maybe a couple of thousand tops, I thought. (By the way, the Metropolitan Police, who I rang to check, say they don’t give out estimates of crowd sizes, and instead rely on the organisers to give them a number.) But there were very few people on Whitehall. By the entrance to Downing Street, I saw just two people lamely holding placards, outnumbered by bored-looking police by about 10 to one. The police helicopter circling overhead, as I tweeted at the time, seemed completely over the top for such a small protest (costing the taxpayer about £850 an hour). It wasn’t like I was pushing through crowds: I was walking along quite empty pavements.

Imagine my shock, then, when I got home and saw a report from The Hill going viral on Twitter. It began: 'Hundreds of thousands of protesters rallied outside the British Parliament, as the governing body debated whether it would invite President Trump for an official state visit.' It went on: 'About 300,000 protesters had gathered at Parliament Square on Monday...'

300,000?! There was no way around it: this was fake news breaking in front of my eyes. What irony that it was about an anti-Trump crowd size. But this was not a marginal error. The report was wrong by a factor of about 100. (The Hill's report cited CNN, though it’s not clear if that was another error from The Hill – or if CNN got the number wrong first.)

Suddenly, and to my surprise, I felt a surge of sympathy for Donald Trump. Is this what he's talking about? Journalists who are so eager to overstate the opposition to him that they are either willing to make stuff up, or sloppily overlook such an obvious error. Just to be clear, this was not a detail buried low down in the story. It was the intro (the lede, as American journalists call it) – the whole point of publishing the thing.

As well as getting the crowd size wrong by, oh, about 298,000, it’s also utter nonsense to say Parliament’s ‘governing body’ was deciding whether President Trump should have a state visit later this year to the UK. Yes, the debate involved members of parliament and was the result of two petitions – one against the state visit, which received 1.85 million signatures, and one in favour of it, which received 311,000.

But it was in Westminster Hall, not the chamber of the House of Commons. It was an opportunity for MPs to vent and grandstand, not vote. The Prime Minister’s invitation to Donald Trump for a state visit is not going to be rescinded in a million years. The Queen will host the President of the United States in 2017.

Later, even when The Hill's story had been corrected to say ‘about 7,000 protesters’ had been in Parliament Square – still an exaggeration in my opinion – there was no apology, or explanation of the massive error in reporting. Just a sterile little note: ‘Updated 5:50pm’. The Hill’s tweet, referring to hundreds of thousands of protesters, is still live today as I write this. The article still refers wrongly to the ‘governing body’ debating ‘whether it would invite President Trump for an official state visit’.

No wonder Donald Trump is making fake news a key theme of his early presidency. When he says he is unfairly treated by the US media, he has a point. If he doesn't have a point, how do you explain The Hill's story?