Peter Tatchell, an Australian-born political campaigner and member of the Green Party of England and Wales, criticized the queen for putting "royalty before human rights," The Independent reported Sunday.
Prince William and Kate Middleton will be married Friday at Westminster Abbey. William is the eldest son of the late Princess Diana.
Tatchell was angered over the invitation of the crown prince of Bahrain, whose Persian Gulf state has suppressed pro-democracy protests in recent months.
"It's deplorable that the queen has invited royal dictators from Bahrain, Swaziland and Saudi Arabia who preside over severe human rights abuses, including detention without trial, torture and violent repression of protests," Tatchell said.
Missing from the guest list were former Prime Ministers Tony Blair and Gordon Brown, both of the Labor Party. Their conservative predecessors, John Major and Margaret Thatcher, received invitations.
"It appears odd that surviving prime ministers other than Tony Blair and Gordon Brown have rightly been invited to a semi-state occasion, while they haven't," said Michael Dugher, a Labor member of Parliament. "It's a surprising selection. On big occasions such as this one would expect an all-or-nothing approach."
A spokesman for the queen said in The Sunday Telegraph, "There is no protocol reason to invite them, so unless they [the couple] wanted to invite former prime ministers for a personal reason, there is no reason to do so."