London (CNN) -- Rights groups have slammed a lunch hosted by Britain's Queen Elizabeth II on Friday for including monarchs criticized for their countries' human rights records.
Among those who attended the event at Windsor Castle is the king of Bahrain, whose government has come under fire for its crackdown on anti-government protests last year and its handling of continuing unrest.
Swaziland's King Mswati III, accused by critics of enjoying a lavish lifestyle at public expense while his people suffer great poverty, was also there.
The monarchs' lunch, with a menu that included English asparagus, lamb from Windsor and strawberries from Kent, is part of celebrations of the Queen's Diamond Jubilee, marking her 60 years on the throne. It will be followed by an evening banquet at Buckingham Palace hosted by Prince Charles and the Duchess of Cornwall.
Human rights campaigner Peter Tatchell said it was "outrageous that the queen has invited royal tyrants to celebrate her Diamond Jubilee."
He called for the invitations to monarchs heading nations with poor records on rights to be withdrawn immediately and said protesters would stage a rally outside Buckingham Palace as royals arrive for the dinner.
"The invitations are a shocking misjudgment," he said. "They show the queen is out of touch with the humanitarian values of most British people. She's putting royalty before human rights."
No significant protests marked the monarchs' arrival at Windsor, where they were each greeted by the queen and the duke of Edinburgh before sitting down to eat.
But rights group Amnesty International said the event risked giving the impression that Britain did not care about the victims of human rights abuses.
"We can imagine why victims of recent human rights abuses in countries like Bahrain, Saudi Arabia, Swaziland and Thailand might feel their plight has been ignored when they see the guest list for the queen's diamond jubilee celebrations," said Philip Luther, Middle East and North Africa director for Amnesty International.
He said that Bahrain's King Hamad al-Khalifa had tried to address the abuses committed last year by setting up an independent inquiry but that failures to ensure reforms are fully implemented "make his promises seem hollow."
Luther added: "He should use this occasion to commit to releasing the prisoners of conscience still held in Bahrain and ensure true accountability for the violations of the last year."
The UK Foreign Office said all the world's sovereigns have been invited to the lunch, which was organized by the Royal Household with its support, although all will not necessarily attend.
A Foreign Office spokesman defended the inclusion of al-Khalifa at the lunch, saying Britain and Bahrain "work together closely on a range of important issues."
"The UK is a longstanding friend and ally of Bahrain, and ministers regularly meet with Bahraini counterparts in the UK and abroad," he said.
"This strong relationship also allows us to have a full and frank discussion on a range of issues including those where we have concerns. On human rights, we support the reforms already under way in Bahrain, and we want to help promote that reform."
The reforms include "bringing to account those individuals responsible for human rights abuses," he said.
Grand Prix organizers were urged to cancel the F1 race in Bahrain last month because of concerns about human rights.
Bahrain is home to the U.S. Fifth Fleet and is viewed as an important bulwark against Iran's influence in the Persian Gulf.
Bahrain's king is not on the guest list for the smaller evening event, but Swaziland's monarch is among those invited to both lunch and dinner. The menus for the two meals have not been made public.
Royals from Denmark, Luxembourg, Norway, Sweden, Monaco, Brunei, Jordan, Saudi Arabia and Thailand are among those invited to both events, as are the emperor and empress of Japan.
The Spanish queen did not attend, however, amid tensions over a planned visit by Prince Edward and his wife next month to Gibraltar, a UK territory that Spain also claims.
"Queen Sofia has canceled her visit to the UK for the Jubilee celebrations following the advice from the Spanish government that the visit would be inappropriate in the current circumstances regarding Gibraltar," a spokesman for the Spanish Royal Household said.
CNN's Max Foster, David Wilkinson and Laura Smith-Spark contributed to this report.