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.
"I will be raising concerns about human rights when I meet him".
The crown prince has been the driving force behind a modernisation programme, Vision 2030, in Saudi Arabia - but the reforms have been dismissed as a "mirage" by campaigners.
Meanwhile, May, who visited Saudi Arabia in November, will be keen to capitalize on the Crown Prince's visit to strengthen trade and security ties ahead of Britain's departure from the European Union.
Buses have spent two days touring London with banners accusing Prince Mohammed of war crimes, with more planned for Wednesday before the main rally, and parliament called an urgent debate on the visit.
Mohammed bin Salman was received by the monarch at the start of a trip that has brought Saudi Arabia's role in the bloody civil war in Yemen into sharp focus. "It is definitely not ethical, bearing in mind what is going on in Yemen every single day, every single second, even as we speak".
Saudi Arabia has waged a long military campaign in Yemen in support of the internationally recognized government that Houthi rebels drove out of the capital, Sanaa.
The three-year conflict, which began with the Saudi-led intervention to fight Iran-backed Huthi rebels, has left 22.2 million people dependent on food aid, according to United Nations figures.
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.
The handout picture provided by the Saudi Royal Palace on March 7, 2018, shows Saudi Crown Prince Mohammed bin Salman shaking hands with Britain's Queen Elizabeth II at Buckingham Palace in London.
Turkey has also worked with Iran, Saudi Arabia's arch-rival in the Middle East, to try to reduce fighting in northern Syria in recent months, and Iranian and Turkish military chiefs exchanged visits a year ago. London also eyes listing state oil firm Saudi Aramco in its stock market.
The advertisement appeared next to an article by Labour MP Emily Thornberry criticising the government for "rolling out the red carpet" for the ruler.
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 past year.
"Behind the smiles and handshakes on show tomorrow, and despite Prince Mohammed bin Salman's attempts to be a poster boy for progress, there are some uncomfortable truths about his repressive government that must not be forgotten", she wrote in an editorial for The Independent.
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.
And in an extraordinary allegation, Mr Corbyn told Theresa May: "It can not be right that her Government is colluding in what the United Nations says is evidence of war crimes".
However, the United Kingdom is selling arms to the Saudis which are used in the war in Yemen, which has turned into one of the world's largest humanitarian crises as aid routes are strangled and record numbers of children have cholera.
The UK has, so far, sold more than six billion pounds of arms to Saudi Arabia.