A small group of around 25 Bahrainis who now live in London have protested outside the Bahrain embassy in London over the presence of their king at the Queen Elizabeth II's Diamond Jubilee lunch for foreign royals at Windsor Castle.
The protesters call King Hamad bin Isa Al Khalifa the "Butcher of Bahrain".
They held up banners and placards on Friday depicting blood running from the mouth of the King alongside photographs of Bahrainis who have been badly beaten.
The Bahraini government stands accused of human-rights abuses during months of protests by the majority Shia Muslim population against the ruling Sunni Muslim Al Khalifa family.
The protesters said they could not understand why Buckingham Palace included Hamad on Queen Elizabeth's guest list the jubilee lunch.
"It's very strange that the United Kingdom has invited such a dictator after all his crimes in Bahrain, to be part of a lunch with the Queen. We believe he should not be invited," said Ali al-Fayez, who described himself as a political activist who has been living in the UK for the past year.
"He should be behind bars; he should be on the wanted list of the United Kingdom, not invited to Buckingham Palace."
Later on Friday, the foreign royals attended a banquet dinner at Buckingham Palace as the guest of Prince Charles.
Britain and Commonwealth states are celebrating 60 years of Queen Elizabeth's reign in a series of events in the UK and elsewhere.