A few dozen demonstrators chanted slogans and held banners, while images of Trump alongside Jeffrey Epstein and his 2023 mug shot were projected onto the castle walls.
Thames Valley Police said four people were arrested on suspicion of malicious communications in connection with the projections. A senior officer said a full investigation was under way in cooperation with partner agencies.
Trump arrived in the UK for a two-day state visit that blends diplomacy with royal ceremony. Events include a banquet, horse-drawn carriages and military honour guards. King Charles III will host him at Windsor Castle before talks with Prime Minister Keir Starmer at Chequers.
The British government has framed the visit as a demonstration of the strength of the trans-Atlantic alliance and an opportunity to advance a multibillion-dollar technology deal. U.S. officials said they expect the trip to highlight shared values and to mark the approaching 250th anniversary of American independence.