Protestors gathered outside 10 Downing Street late Friday, demanding an end to the war in Yemen and the British government's support of Saudi Arabia.
The demonstrations were organised by the Stop The War Coalition and Hands Off Yemen anti-war movements.
Protestors and campaigners held placards reading "Hey, hey, Theresa May, how many kids have you killed today?" and chanted slogans such as "Stop arming Saudi! Stop bombing Yemen!" and "Justice for Jamal!".
"We believe that the murder of Jamal Khashoggi demonstrates the nature of the Saudi regime," a senior campaigner of the Stop The War Coalition said.
"A regime that is capable of the arbitrary murder of a journalist and pretending the murder didn't happen is a regime that is [also] capable of terrible warcrimes in Yemen and pretending that they didn't happen either."
The campaign activist was skeptical that the British government would apply significant pressure on Saudi Arabia toaccelerate a peace process in Yemen and said if Prime Minister Theresa May wanted to influence the Saudi monarchy, she should withdraw UK support for the Saudi military command.
"Unfortunately, we believe the British government is drifting and not applying pressure, so we believe British complicity with this war must end, and that is why we are here, to demonstrate our opposition and join the growing international movement which is demanding an end to the war in Yemenand the introduction of a peace process."
The activists also demanded justice for Saudi journalist Jamal Khashoggi, who was brutally murdered in the Saudi Arabian consulate in Istanbul on October 2.
"We cannot let the world forget about Jamal," one protestor said, adding that forgetting about him "would give the Saudi regime a green light to continue their Machiavellian and reckless acts throughout the region without any fear of accountability".
In an interview with Sky News, May said she planned to speak to Saudi Crown Prince Mohammed bin Salman, who has come under heavy criticism and pressure from the international community following the murder of Khashoggi by Saudi agents in Istanbul, with the international community accusing bin Salman of ordering it.
"I am going to speak to the Crown Prince of Saudi Arabia, but it is the relationship we have with Saudi Arabia that enables me to sit down with him and be robust on our views on two issues.
"First of all, the terrible killing of Jamal Khashoggi, and the message I will be giving and the message we have given from the UK from the time it happened is that the Saudi Arabians need to ensure that their investigation is a full investigation, that it is credible, that it is transparent and that people can have confidence in the outcome of it and that those responsible are held to account.
"But I will also be raising the situation inYemen, where the humanitarian crisis is getting worse. We are very concerned about that."
She added that now is the time and there is an opportunity to come to a political solution because it will ensure a safe and secure future for the people of Yemen.
An estimated 8.4 million people in Yemenare at risk of severe famine and more than 22 million people, or 75 percent of the population, are in need of humanitarian assistance.
Impoverished Yemen has been wracked by conflict since 2014, when Houthi rebels overran much of the country, including the capital, Sanaa.
The conflict escalated in 2015 when Saudi Arabia and its Sunni Arab allies launched a military campaign aimed at rolling back Houthi gains in Yemen and shoring up the country's Saudi-backed government.
The war has resulted in a collapsed economy and a cholera outbreak that has affected over 1.1 million people.
Riyadh has repeatedly accused the Houthis of acting as a proxy force for Iran, Saudi Arabia's arch-foe in the region.