In response to lawmakers' questions Wednesday about United Kingdom arms deals with Saudi Arabia and the use of UK-made weapons in Yemen, May said she would raise concerns over the humanitarian crisis during talks with the Crown Prince.
She added: "Their involvement in Yemen came at the request of the legitimate government of the Yemen, it is backed by the UN Security Council and as such we support it".
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.
May defended the red-carpet treatment for the crown prince, saying the close counter-extremism alliance with the Saudis had saved hundreds of lives in Britain.
"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.
Saudi Crown Prince Mohammed bin Salman has arrived in London for his controversial three-day visit amid massive outcry and protests against Riyadh's human rights violations and its deadly war on Yemen. "The British government pretends to care about human rights and war crimes but when it comes to Saudi Arabia in Yemen there is nothing but a shameful silence".
"The pictures of the crown prince on the steps of Downing Street will be projected around the world, and will be used by the regime to legitimize it on the world stage". She made no mention of the 200 elites he ordered detained in the Ritz Carlton hotel in a purported corruption crackdown.
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The change comes amid uproar over two recent cases of men accused of having sex with 11-year-old girls. Ms Schiappa told AFP news agency that her ministry was "very glad" that the higher age was chosen.
The trip, which also includes stops in Egypt and the United States, is seen as part of an effort by the Crown Prince - considered a key power player behind the King and a reformer by Saudi standards - to "rebrand" Saudi Arabia and show its openness to change.
Campaigners against the war also rallied near parliament and held a protest outside the gates of Downing Street.
Britain is already a major supplier of arms to Saudi Arabia but post-Brexit will increasingly have to compete with European arms industry.
Although May noted that she would raise humanitarian concerns about Yemen in his meeting with bin Salman, she stressed that all arms sales to Saudi Arabia were strictly regulated.
A spokesman for the PM said she will "acknowledge the steps" taken recently by Saudi Arabia to address the crisis, but will "stress the importance of full and unfettered humanitarian and commercial access" through the ports of Hodeidah and Salif, which have previously been hit by coalition blockades.
The debate took place as Prince Mohammed lunched with the British monarch on the first leg of trip packed with displays of diplomatic affection created to help the widen longstanding defence ties into a more far-reaching partnership.
"When she meets the Crown Prince, Theresa May should urge him to commute the sentences of all child protesters facing execution".
His reported comments reflect Saudi Arabia's deep suspicion of President Tayyip Erdogan, whose ruling AK Party has its roots in Islamist politics and who has allied his country with Qatar in its dispute with Saudi Arabia and some other Gulf states.