He kicked off his visit Wednesday, lunching with Queen Elizabeth at Buckingham Palace.
He is expected to be given the full red carpet treatment during his thee-day stay including dinner with Prince Charles and clearance to attend top level foreign policy briefings.
There were crowds of protesters and supporters of the royal visit on Downing Street, being separated by police.
It is also reported that Britain is a military supplier of Suadi Arabia.
Her statements were interrupted briefly as opposition lawmakers cried "Shame!"
She said: "The Prime Minister raised our deep concerns at the humanitarian situation in Yemen".
This builds on the International Development Secretary's visit to Saudi Arabia in December, where she called on all parties to the conflict in Yemen to restore and maintain full access for humanitarian and commercial imports and find a peaceful solution to stop Yemen falling into starvation.
Bin Salman's plan has also meant sharper competition with regional rival Iran for influence in the Middle East-leading Saudi Arabia to threaten war in countries such as Lebanon.
Human rights groups have blamed the Saudi government for the destruction of infra-structures such as health centers and schools. The military aggression has so far killed over 13,600 Yemenis.
This year Forbes US chose to abstain Saudi Arabia billionaires from the list due to reports revolving around their asset seizures.
Critics say Britain has been slow to condemn rights abuses by Saudi Arabia, a key regional ally and major purchaser of United Kingdom -made weapons. The British government seeks to turn its historic defense relations into trade and investment, focusing both on the expanding Saudi Arabian export market in the services sector and on attracting Saudi money to finance domestic projects.
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.
Mohammed also is planning to visit the United States, home to the New York Stock Exchange, for an investment-focused visit set to start March 19.
Diplomats say the agreements could be worth more than $100 billion. London taxis display advertising graphics welcoming Prince Mohammed and electronic billboards promote the visit.
"It can not be right that her Government is colluding in what the United Nations says is evidence of war crimes". More similar programs are underway before the main planned rally on Wednesday.
O'Donnell said Wednesday it took almost two years of requests, including a meeting with the crown prince at the Saudi embassy in the United States, before he agreed to the interview.
Since coming into power in the summer of 2016, leapfrogging his older cousin, Mohammed bin Nayef, MBS has grabbed, consolidated and inherited much of the power over the kingdom, still officially ruled by his father, King Salman, 81.
The U.N. said Saudi Arabia was at the center of the worst humanitarian crisis since World War II. In a blog post, the firm's founder highlighted the significant changes he has seen in recent years in Saudi Arabia, such as a new inclusion of Saudi women in public life. "Theresa May is handing a major propaganda coup to the dictatorship".
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