(CNN) US President Donald Trump arrived for a controversial two-day visit to the United Kingdom on Thursday, declaring he was popular with Britons because of his hardline stance on immigration and questioning Prime Minister Theresa May's Brexit strategy.

Shortly before leaving a turbulent NATO summit in Brussels , Belgium, Trump said the British people shared his concerns on immigration, claiming "that's why Brexit happened."

"She should negotiate the best way she knows how, but it's too bad what's going on," the President said.

Trump told The Sun he felt her approach of keeping some ties to the EU would "affect trade with the United States, unfortunately in a negative way."

He told the newspaper he felt the UK was "losing its culture" because of immigration.

His comments were a signal that his trip to the UK -- billed as a "working" visit but laced with a degree of pomp and ceremony -- would be eventful. He is due to hold talks with May and take tea with Queen Elizabeth II. His schedule has been carefully designed to avoid planned protests, including the flying of a giant "baby Trump" balloon

Donald Trump and first lady Melania Trump step off Air Force One as they arrive at London's Stansted airport.

Trump arrived at Stansted Airport, north of London, early Thursday afternoon. He was flown by helicopter to the residence of the US ambassador to the UK at Regent's Park, central London, where a large steel security fence has been erected.

Trump is spending the night there after a black-tie dinner at Blenheim Palace , the birthplace of one of his heroes, Winston Churchill.

Speaking at the dinner, May repeated her desire to conclude a post-Brexit trade deal with the United States.

"As we prepare to leave the European Union, we have an unprecedented opportunity to do more. It's an opportunity to reach a free trade agreement that creates jobs and growth here in the UK and right across the United States," she said, in remarks released by Downing Street.

"It's also an opportunity to tear down the bureaucratic barriers that frustrate business leaders on both sides of the Atlantic." She mentioned thousands of US companies having offices in the UK. "And the strength and breadth of Britain's contribution to the US economy cannot be understated. The UK is the largest investor in the US, providing nearly a fifth of all foreign investment in your country," she said.

UK Prime Minister Theresa May, US President Donald Trump and his wife Melania arrived at the black-tie dinner at Blenheim Palace on Thursday evening.

Security is tight for Trump's visit. London's Metropolitan Police department has drafted in reinforcements from almost every police force in the UK, Home Office Minister Nick Hurt told Parliament on Thursday. He said it was a "significant" policing operation but that the right to protest was "a fundamental one and needs to be respected."

Asked whether he was concerned about protests during his visit, Trump told reporters in Brussels: "I think they like me in the UK. I think they agree with me on immigration." He reminded reporters that his mother was from Scotland, which he plans to visit at the weekend.

On Thursday evening, hundreds of demonstrators gathered in Regent's Park in protest. The protesters blared bells and whistles, chanting "Stay out, stay clear, Donald Trump get out of here" and booed as the US President left for Blenheim Palace in a helicopter with his wife.

Trump also issued a warning to the European Union, telling it to "be very careful" of increasing immigration into the region, as he described a global backlash to migration.

"I ... partially won an election because of immigration," he said, also pointing to Italy's recent vote, in which Giuseppe Conte took leadership of the government after campaigning on a populist anti-immigration platform.

Trump weighed in on British political affairs. "I'm going to a pretty hot spot right now, right? With a lot of resignations," he said, in a reference to recent turmoil in May's government.

Asked for his views on Britain's departure from the European Union, Trump said he did not wish to be seen as interfering in UK politics. But he said: "I would say Brexit is Brexit," echoing the "Brexit means Brexit" phrase often used by May.

"The people voted to break it up so I imagine that is what they would do, but maybe they're taking a different route," he added, apparently referring to the UK government's recent decision to pursue a closer relationship with the European Union after Brexit. "I'm not sure that's what they voted for," Trump said.

In other remarks to The Sun, Trump said former Foreign Secretary Boris Johnson would make a great prime minister. "I think he's got what it takes, and I think he's got the right attitude to be a great prime minister."

Trump postponed his trip to the UK several times as a petition to block him from entering the country gained so many signatures it was debated in Parliament. The Parliament speaker made clear that Trump would not be invited to address MPs.

The petition against Trump said an official state visit would be embarrassing for the Queen.

He again postponed his visit in January this year after remarking that the relocation of the US Embassy in London was a "bad deal" made by President Obama, and that he was not interested in cutting the ribbon at the opening.