Protesters chanted “Hands off Yemen - stop the bombing now” and “Stop the killing" as they demanded the UK halt arms sales to the Gulf state, which has been criticised over its bombing campaign in Yemen.
Labour’s Shadow International Development Secretary, Kate Osamor, and Green Party Deputy Leader Amelia Womack were among the speakers.
"If he says he truly is a progressive leader then he needs to look at what makes a society progressive." Ms Osamor told The Independent.
"Now, that's just in Saudi Arabia, so women should be equal to men without a shadow of a doubt. if that's not happening then he's got a lot of work to be doing."
"On top of that, more importantly, I'd be talking [to the Crown Prince] about the Yemeni people, who at this point in time are being starved," she added. "He must take this away and make differences. if he doesn't, then he's not welcome back."
Nouria Nagi, a British-Yemeni woman, said she had lost five relatives in the conflict, which has to date caused the deaths of at least 10,000 people. She said she had joined the protest to tell the Crown Prince: “Use your brain – stop the killing”.
"Five of my family have died because of this war," she said. "There is nothing [in Yemen]. If you have no money, you die.
"Nobody is getting any help or support and everybody is quiet about it. Nobody is doing anything."
Ms May will hold further talks with the Saudi leader at Chequers tomorrow, with the Crown Prince also set to meet the Queen, Prince Charles and Prince William during his three-day visit.
Saudi Arabia’s foreign minister, Adel al-Jubeir, had earlier claimed the protesters did not understand his country’s role in Yemen.
“I believe [opposition group’s] positions are based on misunderstanding and not knowing,” he told the BBC. “They criticise us for a war in Yemen that we did not want, that was imposed on us.
“They criticise us for a war in Yemen that is a just war, that is supported by international law."
No 10 said Ms May had raised the issue of human rights and the situation in Yemen with the Crown Prince.
A Downing Street spokesperson said: “The Prime Minister welcomed recent reforms in Saudi Arabia, including on women attending sporting events and the cinema, and being legally able to drive from June.
“The Prime Minister and Crown Prince agreed that we should continue working together to explore ways the UK can support Saudi Arabia to progress and intensify these reforms, particularly on women’s rights, and on universal human rights, where the Prime Minister noted our particular concerns in the case of Raif Badawi.
“The Prime Minister raised our deep concerns at the humanitarian situation in Yemen. The Prime Minister and Crown Prince agreed on the importance of full and unfettered humanitarian and commercial access, including through the ports, and that a political solution was ultimately the only way to end the conflict and humanitarian suffering in Yemen.”
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