Protests are expected to greet Bahrain's King Hamad bin Isa Al-Khalifa when he arrives in London on Friday for engagements with Britain's royal family.
London police said they are expecting demonstrations when the king attends a conference on Friday to strengthen British-Bahraini ties, which will also be attended by Queen Elizabeth II's son Prince Andrew.
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Protesters angry about the treatment of anti-regime activists in Bahrain are also expected to be present when he joins the Queen at Sunday's Royal Windsor Horse Show, west of London.
The king will be accompanied by his son, Prince Nasser bin Hamad, who faces a London court bid to overturn his immunity from prosecution over torture allegations.
The case was brought by a Bahraini refuge, who claims the prince tortured anti-regime demonstrators during a crackdown against the uprising in 2011.
The Bahraini embassy in London called the claims "unfounded, false and politically motivated", according to the London Times.