There will be at least one royal presence at Friday's wedding that is likely to raise eyebrows. Among the 46 foreign royals seated in the south lantern, just behind the British monarchy, will be Bahrain's crown prince, Salman bin Hamad al-Khalifa, an invitation that could prove awkward in light of his government's brutal treatment of mainly Shia pro-democracy protesters.

At least 30 people have died in Bahrain since protests began in mid-February, including four who died in official custody, and many activists and lawyers have been imprisoned.

Human rights campaigners are already petitioning the foreign secretary, William Hague, to revoke the invitation, saying the prince should not be allowed to attend given the bloodshed in his country. Earlier this week, the crown prince praised the "relentless efforts of Bahrain's security forces to maintain security and stability".

"The reform and modernisation process, initiated by His Majesty King Hamad, is continuing to serve every citizen," he added.

Mehdi Hasan, senior editor (politics) at the New Statesman, tweeted: "Crown Prince of Bahrain to attend Royal Wedding. Royals & their apologists should be ashamed."

The crown prince, seen as a moderate reformer, has been a guest of the royal family before – in December 2004, Prince Charles invited him to St James's Palace and the pair have had regular discussions on relations between their two countries, according to recent reports in the Saudi press.

The crown prince said earlier this month that the ruling family was committed to reform, but unrest had escalated to the point that security forces had to step in. "I will continue … to be firm on the principle that there can be no leniency with anyone who seeks to split our society into two halves," he told Bahraini TV.

In March, Bahrain's Sunni rulers announced martial law, deployed security forces and called in troops from neighbouring Sunni-led Gulf Arab countries, including Saudi Arabia.

"Bahrain has created a state of fear, not a state of safety," said Joe Stork, deputy Middle East director of Human Rights Watch.