Prime Minister Theresa May defended Britain's links to security ally Saudi Arabia on Wednesday, as Crown Prince Mohammed bin Salman met Queen Elizabeth for lunch on a high profile to London that has drawn protests over Riyadh's human rights record.
A fiery exchange in parliament between May and opposition Labour Party leader Jeremy Corbyn underlined tension in Britain over the trip, which is aimed at building a broader economic partnership between the two countries, but has sparked protests about human rights and the war in Yemen.
"The link that we have with Saudi Arabia is historic, it is an important one, and it has saved the lives of potentially hundreds of people in this country," May said, pausing her answer briefly as opposition lawmakers cried "Shame!."
The debate took place as Prince Mohammed lunched with the British monarch on the first leg of a trip packed with displays of diplomatic affection designed to help widen long-standing defense ties into a more far-reaching partnership.
May later met Prince Mohammed, extending a warm diplomatic welcome to the conservative kingdom's heir apparent.
Britain is looking for trading partners as it exits the European Union, and energy powerhouse Saudi Arabia needs to convince skeptical investors about its domestic reforms.
But demonstrators gathered outside May's office to protest both countries' roles in Yemen where war has killed an estimated 10,000 people.
"I don't believe that someone like Mohammed bin Salman should be welcomed in Britain," said Hassan Yassine, a 25-year-old customer service worker from London. "It is definitely not ethical, bearing in mind what is going on in Yemen every single day, every single second, even as we speak."
Earlier, Corbyn said British military advisers were "directing the war" in Yemen. May's spokesman later said British personnel had no role in carrying out air strikes, were not involved in directing or conducting operation in Yemen, and were not involved in Saudi targeting decisions.
Police said a man was arrested on suspicion of criminal damage after an egg was thrown at police vehicle as bin Salman's motorcade arrived in Downing Street.
Wednesday's first official engagement was a trip to Buckingham Palace to see Queen Elizabeth - a rare honor usually reserved for heads of state.
The Saudi delegation then met May and senior ministers inside May's Downing Street offices to launch a UK-Saudi Strategic Partnership Council - an initiative to encourage Saudi Arabia's economic reforms and foster cooperation on issues such as education and culture, as well as defense and security.
Britain is vying to land the stock market listing of state oil firm Saudi Aramco, but no decision is expected this week.
"We would like the Aramco share option to be issued in the United Kingdom and we will continue to suggest the City would be the best place for it," junior foreign office minister Alistair Burt told parliament.
Later this month Prince Mohammed visits the United States, which also wants the lucrative listing, although sources said both countries may miss out.
British officials were privately delighted at the decision by Prince Mohammed, 32, to choose Britain as the major western destination on his first foreign trip since becoming heir to the Saudi throne last year.
The British government is keen to develop a two-way trade and investment relationship, eyeing both an expanded market in Saudi Arabia for service sector exports, and attracting Saudi cash to finance domestic projects.
Saudi's ambitious heir wants to show that "shock" reforms have made his country a better place to invest and a more tolerant society.
Photographs posted online showed London taxis displaying advertising graphics welcoming Prince Mohammed, and electronic billboards promoted pro-Saudi messages with the hashtag #anewsaudiarabia.
Business deals are possible with British defense group BAE Systems and European weapons maker MBDA, and initial agreements could be concluded on gas exploration, petrochemicals and other industries, according to British and Saudi sources.
The three-day visit will include a second Royal audience - dinner with the Prince Charles and Prince William - a briefing with national security officials, and a prestigious visit to the prime minister's country residence.
May intends to use the private dinner at Chequers on Thursday, a 16th-century manor house 40 miles (60 km) northwest of London, to bring up concerns over the humanitarian crisis in Yemen, her spokesman said.
A Saudi-led military coalition is fighting the Houthi movement in Yemen, generating what the United Nation said in January was the world's worst humanitarian crisis.
Demonstrators are protesting against Britain for licensing 4.6 billion pounds of weapons sales to Saudi Arabia since 2015.
May said all arms sales were strictly regulated, that Saudi Arabia's involvement in the conflict was backed by the U.N. Security Council and her government supported it.