He added: "I didn't see the protesters until just a little while ago. And it was a very, very small group of people put in for political reasons. So it was fake news."
Organisers estimated 75,000 people hit the streets - fewer than those at the anti-Trump rally in 2018, which organisers said drew more than 100,000.
But the protesters were vocal, their chants of "Say it loud, say it clear, Donald Trump's not welcome here" and "Donald Trump, shame on you" ringing in the air as reporters headed to the news conference in the on-off rain.
"We come today because we don't like the tide of right-wing populism that's going across the Western world, especially in America and Europe," said Cat Thorneycroft, 35, an illustrator.
"I don't even necessarily mind that he's had a state visit, but if he's going to come, then this is what's going to happen," she said, referring to the protesters behind her in Parliament Square.
Jeremy Corbyn, leader of the Labour Party, was among those who addressed the crowds. He condemned Trump over his tweets calling London Mayor Sadiq Khan, a longtime Trump foe on Twitter, a "stone cold loser." Corbyn said he was "proud that our city has a Muslim mayor."
Trump also denounced Corbyn, calling him a "negative force."
The Labour leader also said he would resist any attempts to allow private American companies to take over "our precious, wonderful National Health Service . . . We will not stand for that."
Trump tweeted that he had not "seen any protests yet, but I'm sure the Fake News will be working hard to find them."
The world's most famous helium-filled balloon - the 6m-tall blimp depicting a nappy-clad "baby Trump" holding a cellphone - was back, hovering above the scene in Parliament Square.
Kevin Smith, one of the organisers or "Trump babysitters," said it was a "very effective way to prick the pomposity and ego of Donald Trump."
Asked whether it wasn't a rather juvenile form of protest, he said it was "part of a long tradition in Britain of political caricature. It's not unlike cartoons in a newspaper - it just so happens to be 3-D and floats in the air."
The organisers of the balloon crowdfunded more than £36,00 which they are giving to groups they describe as "pushing back against the politics of hate and division that are represented by Trump."
There was also a talking Trump robot who sat on a toilet, saying: "You're fake news! I'm a very stable genius!"
The great British tradition of creating witty - and sometimes rude - placards was on full display.
One protester held aloft a sign that read: "British Humour: the gift of a book to an illiterate man - well played Your Majesty." As part of a customary gift exchange at the start of the visit, the Queen gave Trump a book by Winston Churchill on World War II.
Another man was pushing a shopping cart filled with toilet paper featuring Trump's face on it. "Come on down to Trafalgar and get your Donald Trump toilet paper," he said.
The protests come a day after a lavish state banquet hosted by the Queen at Buckingham Palace.
Bryony Doyle, 23, an illustrator and a nanny protesting in Trafalgar Square, said she was "very pleased" that Trump did not "get the red-carpet treatment when he arrived. I know he went for dinner at Buckingham Palace, but he didn't stay there," she said.
The Trumps are staying at Winfield House, the residence of the US ambassador in London, but not Buckingham Palace, which would normally be the case. The palace is reportedly undergoing renovations.
"I think with us and our culture there's only so much we can do without feeling impolite," Doyle said. "I don't think the Queen could actually say no. I think there's a limit to what the royals can do without causing too much controversy. But that's not the case with the public, which is why we are here all day!"
Some wondered whether the protests would have any effect.
Mark Davis, 64, a lawyer from California on holiday, said: "Trump could come here and do virtually anything and there are people back in the States who will support him. They'd think he's being picked on, or this and that. I don't think this changes anything. In the States, you're either for him or against him."
His wife, Monica Richards, 65, a retired employee of Orange County Superior Court in California, said they were nonetheless excited to be among a throng of "like people who have a great hate for Trump. It's an opportunity to show our support for the forces who are against him."
She then unrolled a pink homemade sign, which read: "Impeach now."
