The Queen has been accused of making a catastrophic error of judgment by dining with the King of Bahrain, whose regime is accused of a catalogue of human rights abuses.
The head of state and her family sat down to lunch with the Middle East ruler and other controversial foreign royals as they celebrated her Diamond Jubilee.
The event was supposed to be a rare meeting of monarchs to celebrate the 60-year milestone but it has been overshadowed by strong criticism from campaigners about those invited to the Windsor Castle event.
Guests from controversial regimes include Swaziland's King Mswati III, the former prime minister of Kuwait Sheikh Nasser Mohammad Al-Sabah, who stepped down over a corruption row, and Prince Mohammed Bin Nawaf Bin Abdulaziz Al Saud, Saudi Arabia's ambassador to Britain.
The anti-monarchy group Republic, along with human rights groups, has accused Bahrain's government, dominated by members of its royal family, of orchestrating the violent suppression of pro-democracy protesters last year.
When the country's King Hamad Al-Khalifa arrived at the Castle he was personally greeted by the Queen, who smiled as she shook his hand and the pair laughed as they shared a joke.
Republic's chief executive Graham Smith said: "The Queen cannot hide behind protocol and precedent, this is a crisis of her own making. The British people strongly support the struggle for democracy in the Middle East and around the world - this is a catastrophic error of judgment that has already prompted a fierce backlash.
"The Queen owes a personal apology to all those fighting for freedom in those countries and to the families of those who have died doing so. The Queen's decision to personally invite these tyrants to lunch sends an appalling message to the world and seriously damages Britain's reputation. Thanks to the Queen's misjudgement, her jubilee will forever be associated with some of the most repressive regimes in the world."
Demonstrations are planned outside Buckingham Palace when the foreign guests attend a dinner hosted by the Prince of Wales and Duchess of Cornwall, but the King of Bahrain will not be attending.
The popular uprisings that toppled a succession of dictators across the Middle East last year failed to ignite significant protests in the Arab Peninsula. But in Bahrain there were major demonstrations, with protesters calling for a greater say in government. These were violently put down by Saudi forces called in by the regime.