The king of Bahrain, whose regime has been accused of rights abuses, was among nearly 50 foreign royals at a lunch Friday to mark the diamond jubilee of Britain's Queen Elizabeth II.
King Hamad, whose Gulf island country is in a state of civil unrest following a brutal crackdown on pro-democracy protests, would attend the event at Windsor Castle, British officials confirmed.
The invitations for the biggest gathering of foreign royalty in Britain since last year's wedding of Prince William and Kate Middleton have thrown up several diplomatic headaches.
Spain's Queen Sophia pulled out of the lunch earlier this week due to tensions with Britain over the disputed territory of Gibraltar, while there have been protests in London over the inclusion of Swaziland's King Mswati III.
The monarchs of Brunei, Kuwait, Qatar and Saudi Arabia were all among the guests at the event at Windsor, to the west of London.
Less controversially, Japan's 78-year-old Emperor Akihito, the king of Norway and the queen of the Netherlands will attend.
Buckingham Palace has said that the Foreign Office approved the invitation to King Hamad.
But veteran British lawmaker Dennis MacShane, a former Europe minister, said the queen should not have to "dine with a despot".
The Bahraini regime "has done such terrible things to its own people since the Arab awakening a year ago", MacShane said. "For too long we have turned a blind eye to the repression carried out under the rule of royals in Arabia."
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The Foreign Office "should protect the British queen rather than expose her to having to dine with a despot", he added.
An AFP reporter at Windsor said there was no sign of demonstrations and security was low-key.
Spectators who gathered to see the arrival of the limousines carrying the monarchs said it was a royal celebration that should have nothing to do with politics.
"The Foreign Office did the right thing to invite everybody. I don't think the queen should be drawn into politics," said Robert Charles, 72, a retired factory owner.
The lunch will be followed by a dinner for most of the foreign royals at Buckingham Palace hosted by heir to the throne Prince Charles, although King Hamad is not attending the evening event.
Human rights activists have promised to stage a protest outside Buckingham Palace before the dinner of what they called "dictator monarchs".
Britain has close links to Bahrain and has had to tread carefully in diplomatic terms since the unrest.
Prime Minister David Cameron held a meeting with King Hamad at his Downing Street residence in December when he urged him to implement reforms, and offered Britain's support.
Bahrain's Crown Prince Salman bin Hamad al-Khalifa turned down an invitation to William's wedding in April last year after a public outcry in Britain.
Amnesty International says around 60 people have been killed in Bahrain since the anti-regime protests first erupted in February last year.
On Wednesday, a group of Swazis living in Britain protested outside London's luxurious Savoy Hotel, where King Mswati was said to be staying with an entourage of more than 30 people.
The lunch is one of the main events in the run-up to four days of celebrations on June 2-5 to mark the queen's 60th year on the throne, which will include a pageant on the River Thames featuring more than 1,000 boats.