Britain's Queen Elizabeth II and Prime Minister Theresa May have welcomed Saudi Arabia's Crown Prince Mohammed bin Salman to the United Kingdom.

While the nation rolled out the red carpet as his three-day visit began on Wednesday, protesters took to the streets to draw attention to the kingdom's involvement in the conflict in Yemen, and to highlight human rights issues.

May had said she would raise concerns about human rights and the situation in Yemen, where a Saudi-led coalition has battled Iran-allied rebels since 2015 in a conflict that has claimed 10,000 lives.

But May also praised economic and social reforms introduced by the 32-year-old since he became heir apparent to 82-year-old King Salman last year. Changes include allowing women to drive, and lifting a ban on movie theaters.

"We want to work together to explore ways we can support the Kingdom of Saudi Arabia to progress and intensify these reforms," The Associated Press reported May as saying. "We will also discuss how we can enhance our cooperation in tackling international challenges, including the conflict and humanitarian crisis in Yemen."

The BBC said the crown prince noted there were "huge opportunities" to boost trade with Britain and he insisted the nations must cooperate to deal with "failed states ...terrorist and extremist organizations".

The UK is hoping to capitalize on Saudi plans to open up and diversify its economy and anticipates 65 billion pounds ($90 billion) of mutual trade and investment opportunities during the next decade, something that will be especially important after the UK leaves the European Union.

Metro newspaper reported on Thursday that both supporters of the crown prince and opponents took to the streets as the visit began.

In addition to meeting May in Downing Street, the crown prince was meeting her at a summit on Thursday. He had lunch with the queen at Buckingham Palace on Wednesday and dined with Prince Charles and Prince William. He was also set to meet Defense Secretary Gavin Williamson.