US Secret Service agents bundled Donald Trump from the stage as shots rang out Saturday evening at a media gala, in what the president later described as an attack by a "would-be assassin."

Armed guards opened fire at the gunman who charged through security screening just outside the ballroom of the hotel, where Trump, First Lady Melania Trump, many senior government officials and hundreds of other black-tie guests had gathered.

People dived under tables in chaotic scenes as Secret Service teams swarmed into the black-tie White House Correspondents' Dinner held annually at the Washington Hilton in the US capital.

"A man charged a security checkpoint armed with multiple weapons, and he was taken down by some very brave members of the Secret Service," Trump told a press conference at the White House shortly after the incident.