US President Donald Trump has sparked a backlash from UK politicians by attacking the National Health Service.
In a tweet criticising US Democrats pushing for a universal health system, he said "thousands of people are marching" in the UK because the NHS is "going broke and not working".
This was believed to be a reference to a Save the NHS march on Downing Street on Saturday, demanding more funding.
Downing Street said Theresa May was "proud" of the UK's system.
Her spokesman said the NHS had recently been ranked as the world's best healthcare system and that Health Secretary Jeremy Hunt, who has hit back at the US president on Twitter, "speaks for the government".
Organisers of the demonstration said it had been aimed at showing people's "love" for the NHS.
The chief executive of NHS England said the US president had "got the wrong end of the stick".
In the UK, the NHS is funded out of general taxation, so people do not have to pay when they get treatment.
In the US, when people get treatment they have to pay, most often through health insurance providers.
President Trump's tweet came after ex-UKIP leader Nigel Farage appeared on Fox And Friends, one of the president's favourite shows, talking about the weekend march.
"Dems want to greatly raise taxes for really bad and non-personal medical care," Mr Trump tweeted.
The Democrats are pushing for Universal HealthCare while thousands of people are marching in the UK because their U system is going broke and not working. Dems want to greatly raise taxes for really bad and non-personal medical care. No thanks!— Donald J. Trump (@realDonaldTrump) February 5, 2018
I may disagree with claims made on that march but not ONE of them wants to live in a system where 28m people have no cover. NHS may have challenges but I’m proud to be from the country that invented universal coverage - where all get care no matter the size of their bank balance https://t.co/YJsKBAHsw7— Jeremy Hunt (@Jeremy_Hunt) February 5, 2018
Wrong. People were marching because we love our NHS and hate what the Tories are doing to it. Healthcare is a human right. https://t.co/Pmo2xYSqZh— Jeremy Corbyn (@jeremycorbyn) February 5, 2018
In response, Mr Hunt said that while he "disagreed with claims made on that march", no-one wanted "to live in a system where 28 million people have no cover".
He added: "NHS may have challenges but I'm proud to be from the country that invented universal coverage - where all get care, no matter the size of their bank balance."
Labour leader Jeremy Corbyn - the leader of the UK opposition - also hit back, saying: "People were marching because we love our NHS and hate what the Tories are doing to it."
He added: "Healthcare is a human right."
Analysis by Anthony Zurcher, BBC North America reporter
Donald Trump has said that he doesn't watch much television, claiming once he was too busy "reading documents".
Time and again, however, the president has praised and promoted specific Fox News programmes - particularly the cable network's morning offering, Fox and Friends.
Such was the case on Monday, as the president tweeted about the National Health Service shortly after former UKIP leader Nigel Farage appeared on Fox to criticise the NHS during a segment on a recent London protest march.
Healthcare reform, while a major topic of partisan debate in the US last year, has fallen off the radar recently.
Republicans tried, and failed, to roll back government regulation of the insurance industry instituted by Barack Obama.
In last week's State of the Union address, Mr Trump barely mentioned the topic.
When Fox News catches the president's attention, however, that can change with the flick of a Twitter finger.
NHS funding has been hotly debated in the UK as hospitals struggle to cope with the pressure on resources.
A panel set up by the Liberal Democrats - the fourth largest party in the Commons - has called for a ring-fenced tax to fund the service, saying an extra £4bn is needed for next year and an additional £2.5bn for both 2019 and 2020.
Lib Dem leader Sir Vince Cable urged the US president to read the study, "to find out how to fund a universal healthcare system".
Saturday's demonstration, called NHS In Crisis: Fix It Now, was organised by the People's Assembly Against Austerity and Health Campaigns Together.
In a joint response to the US president, they said people had marched "to show their love for the principles of universal and comprehensive care free at the point of use, paid for through general taxation".
They added: "We don't agree with your divisive and incorrect rhetoric."
Actor and NHS campaigner Ralf Little, who was at the march, told President Trump he was wrong about the reasons for the march.
Hey matey. I was front row of that march and I can assure you that is 100% not why we were there. I can explain properly if you’ve got 5 mins to chat between rounds of golf? https://t.co/slud9VHrAD— Ralf Little (@RalfLittle) February 5, 2018
The UK government says NHS funding "is at a record high" with an extra £2.8bn pledged for the service in the Budget.
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Mr Farage said the NHS was "at breaking point due to a population crisis", adding: "We haven't got enough hospitals, doctors or facilities to cope."
The truth is the NHS is at breaking point due to a population crisis. We haven’t got enough hospitals, doctors or facilities to cope. pic.twitter.com/wG1ux7fi23— Nigel Farage (@Nigel_Farage) February 5, 2018
The British television presenter Piers Morgan, who calls President Trump a friend and who interviewed him last week, did not agree with the tweet:
Wrong, Mr President.— Piers Morgan (@piersmorgan) February 5, 2018
Our NHS is a wonderful, albeit imperfect, health system - and the envy of the world.
By comparison, the US healthcare system is a sick joke & the envy of no-one. https://t.co/Ew5GxclcMF
Simon Stevens, the chief executive of NHS England, responded during an appearance before a committee of MPs.
"President Trump has been tweeting about the National Health Service today, and unfortunately, respectfully, we suggest that tweet got the wrong end of the stick," he said.
"And, in fact, people in this country don't want to ditch our NHS, not withstanding what we've said today, they want to keep it and strengthen it," he said.
Mr Stevens invited the president to visit UK hospitals when he comes to the UK, "and go away understanding that healthcare for everybody delivered at half the cost of the US healthcare system is something that people in this country are deeply and rightly committed to".