"What a beautiful British day this is," President Trump said as he kicked off remarks with thousands of guests gathered outside the White House South Lawn on a rainy morning in Washington.

The royals' state visit is to mark the 250th anniversary of American independence from Great Britain.

"Honoring the British king might seem an ironic beginning to our celebration of 250 years of American independence, but in fact, no tribute could be more appropriate," Trump said. "Long before Americans had a nation or a Constitution, we first had a culture, a character and a creed."

President Trump and King Charles are now reviewing the troops gathered at the White House, after performances by the U.S. Marine Band of "God Save the King" and "The Star-Spangled Banner."

King Charles and Queen Camilla have arrived at the South Portico of the White House for a formal state arrival ceremony. They are being greeted by President Donald Trump and first lady Melania Trump.

Notable officials are gathered on the White House South Lawn in rainy weather for the ceremony that will include a review of the troops, remarks by Trump and more.

President Trump is hosting King Charles III and Queen Camilla at the White House as the "special relationship" between the United States and the United Kingdom is under a microscope amid the war with Iran.

While Trump has repeatedly praised King Charles as a "friend" whom he's known for years, his relationship with U.K. Prime Minister Keir Starmer has been another story. The president has repeatedly assailed the U.K. and other NATO allies over the U.S. and Israeli war with Iran.

Trump said the visit from the king -- the U.K.'s constitutional and ceremonial head of state -- may help restore any damaged ties between the two allies.

"Absolutely. He's fantastic. He's a fantastic man. Absolutely the answer is yes," Trump told the BBC in a phone interview when asked if the visit could help repair the relationship.

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-ABC News' Nicholas Kerr