Kings and queens from around the world have gathered in Britain to mark the Queen's Diamond Jubilee.Twelve UK royals joined the Queen to welcome the sovereigns of 26 countries for a luncheon at Windsor Castle.
Human rights campaigners have condemned the inclusion of Bahrain's King Hamad al-Khalifa but the Palace said the Foreign Office approved his invitation.
Protesters gathered at Buckingham Palace, where Prince Charles is hosting a dinner for some of the royals.
King Hamad is not at the banquet.
Another invitation proving to be controversial is that of King Mswati III of Swaziland, who is accused of living a lavish lifestyle while his people go hungry.
Demonstrators at Buckingham Palace chanted and held banners reading: "Shame on you Liz Windsor," and "Democracy now for Swaziland".
"Inviting bloodstained despots brings shame to our monarchy and tarnishes the Diamond Jubilee celebrations," he said.
Mohammed Sadiq, spokesperson for Justice for Bahrain, told the BBC he fully supported the Jubilee, but did not understand what had changed in Bahrain to prompt the Queen to invite King Hamad.
Graham Smith, chief executive of Republic, said the group was "standing in solidarity" with protesters from Bahrain and Swaziland.Campaigner Peter Tatchell criticised the Queen for "wining and dining dictators who stand accused of very serious human rights abuse".
The Foreign Office earlier said it was having "a full and frank discussion on a range of issues" with Bahrain's government.
The world figures arrived at Windsor Castle in a convoy of black chauffeur-driven cars in time for the start of the lunch at 12:30 BST amid tight security.
Also joining the Queen and Duke of Edinburgh for the lunch were Duke and Duchess of Cambridge, Prince Harry, the Duke of York, Princesses Beatrice and Eugenie and the Earl and Countess of Wessex.
There was a group photograph before the royals sat down to lunch at round tables seating 12. Each group had least one sovereign, their spouse, a member of the British royal family and a member of the royal household.
A British-inspired menu was prepared using many ingredients sourced locally.
To start, the royals were given a tartlet of poached egg with English asparagus.
This was followed by a main course of new season Windsor Lamb with braised potatoes, artichokes, peas, carrots, broad beans, cabbage, and a tomato and basil salad.
Kent strawberries, vanilla Charlotte, dessert fruit and cheese concluded the meal.
The menu for Prince Charles's banquet includes twice-baked cheese souffle with baby leaf spinach, line-caught sea bass with coastal samphire and rhubarb Eton mess.
Mr Tatchell said inviting the kings of Bahrain and Swaziland was "a shocking misjudgement" that showed the Queen was "out of touch with the humanitarian values of most British people".
In April 2011, Bahrain's Crown Prince Salman bin Hamad al-Khalifa pulled out of the Duke and Duchess of Cambridge's wedding amid controversy over human rights.
Mr Sadiq said: "How would the British feel if the Queen had invited Saddam Hussein or Colonel Gaddafi to such an occasion?"
Last month, Bahrain Grand Prix organisers were urged to cancel the race amid public unrest in the country and accusations of human rights abuses.
A Foreign Office spokesman said Britain had "consistently encouraged the Bahraini government to take further urgent steps to implement in full the recommendations of the Independent Commission of Inquiry as His Majesty the King has committed to doing.
"This includes bringing to account those individuals responsible for human rights abuses."
On Wednesday, meanwhile, a group of UK-based Swazis protested outside the Savoy hotel, in London, where King Mswati - who is widely accused of profligate spending - is thought to be staying, with a delegation of 30 officials.
King Mswati is rated by Forbes magazine as the world's 15th richest monarch with a personal fortune of $100m (£62m) - while many of his 1.2 million subjects live in poverty.
Democracy campaigners also want Africa's last absolute monarch to allow political parties and elections.
Saudi and Kuwaiti royals are also attending the banquet.
Amnesty international has recently highlighted repression in Saudi Arabia, as the authorities there crack down on protesters and reformists.
And Human Rights Watch has criticised Kuwait for the suspension of a daily newspaper and the conviction of its editor for incitement.
Meanwhile, Queen Sofia of Spain declined to attend because of a dispute over fishing rights off Gibraltar, a UK territory that Spain also claims.