A man in his 20s was on Tuesday arrested on suspicion of common assault during a walkabout by King Charles.
An egg is believed to have been thrown in the direction of the monarch as he toured Luton town centre.
It is the second time in the past month that eggs have been thrown at the monarch.
In November, a man was detained after throwing eggs at the king and his wife, Queen Consort Camilla, in York.
Bedfordshire Police said the suspect in Tuesday's incident was in custody for questioning after being arrested in St George’s Square in Luton.
The king was temporarily steered away from the crowds outside Luton Town Hall by his security staff shortly after arriving.
He then resumed shaking hands with members of the public after being moved to a different area.
The incident comes as the UK royal family faces fresh drama over Prince Harry and his wife Meghan's Netflix docuseries which will be released on Thursday.
The second trailer for the “Netflix Global Event” was screened on Monday, with Prince Harry speaking of a “dirty game” involving the leaking and planting of stories by the royal family.
He also referenced “the pain and suffering of women marrying into this institution” and a “feeding frenzy”.
During his Luton visit, the king took a ride on the new cable-drawn Dart mass passenger transit system during the Luton visit.
He visited Luton Town Hall and the newly built Guru Nanak Gurdwara during his trip to the Bedfordshire town.
Guests included representatives from the Ghana Society, Royal British Legion, Luton Town’s football academy, veterans and cadets.
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King Charles III in the Prayer Hall with Prof Gurch Randhawa, a member of the Sikh congregation, on a visit to the newly built Guru Nanak Gurdwara in Luton on Tuesday. PA -
King Charles makes the traditional namaste gesture as he speaks to volunteers at Guru Nanak Gurdwara in Luton. PA -
The king unveils a plaque to commemorate his visit. PA -
King Charles speaks to congregation members at Guru Nanak Gurdwara. PA -
The monarch greets local schoolchildren outside Guru Nanak Gurdwara. AP -
King Charles also visited Luton Town Hall. Getty Images -
While at the town hall, the king is shown a photograph of his late mother, Queen Elizabeth II. Reuters -
Council workers wait for the arrival of King Charles. Getty Images -
King Charles greets members of the public at Luton Town Hall. Getty Images
