The Prime Minister's discussions with Mohammed bin Salman also touched on the conflict and humanitarian crisis in Yemen.
The conflict between the Saudi-backed government in Yemen and the Houthis, backed by Iran, is seen by many as part of a regional power struggle between the two countries.
Garry, a 35-year-old teacher who declined to give his last name, agreed.
"This visit is being done to legitimise a brutal dictatorship and to sell arms".
Labour MP Chris Williamson, who was in attendance at the protest, echoed the feelings of protesters during a speech.
He also launched an anti-corruption drive that saw princes, ministers and influential businessmen detained, and which generated an estimated $106bn (£76bn) in settlements.
Only 6 per cent support United Kingdom arms sales to Saudi Arabia; May: "Tell Saudi Prince - stop the slaughter, start peace talks!" were the choice slogans emblazoned on the side of a white van next to the protest, Efe news reported. A Saudi blockade on Yemen has also caused starvation and a widespread cholera epidemic.
The Liberal Democrat leader, Vince Cable, condemned ministers for rolling out the red carpet and providing the equivalent of a state visit to "a dictatorial head of a theocratic, medieval regime".
Bin Salman has a "2030" vision for Saudi, he says, and wants to modernise the desert country, starting by lifting the ban on women driving.
"The UK has been blocking any serious criticism of Saudi Arabia in the UN Security Council and will be offering some ideas to the crown prince who is hoping for a face-saving opportunity to close this file", says a NY based Arab diplomat.
Is the crown prince aiming at bringing in foreign as well as political capital?
Saudi Arabia's Mohammed bin Salman is "not really" a reforming leader and the United Kingdom should immediately stop selling arms to the country, according to shadow foreign secretary Emily Thornberry.
However, activists have warned against interpreting these reforms as heralding new freedoms.
According to analysis by human rights charity Reprieve, executions have doubled under the Crown Prince.
"This vindicates the engagement that we have with Saudi Arabia, to be able to sit down with them", she said.
"It's not true they are directing the war in Yemen".
Most reports focussed on business opportunities for the two kingdoms as a result of "increased ties", as well as "Syria and Yemen, Iran's role in the region, and the role of the global coalition against the [Isis] terror group", the Saudi Gazette said.
The Downing Street spokesperson insisted May had raised the issue of Yemen with the crown prince.
"He's going to War rooms with Boris after tea with Theresa and then onto dinner with (Prince) Charles with photographs in Morning room", the post read.
He accused Britain of failing to hold the Saudi government to account.
Earlier Emily Thornberry, the foreign affairs spokeswoman for Labour, the UK's main opposition party, urged Mrs May to use the UK's influence to call on Saudi Arabia "to stop" the bombing in Yemen. For the last three years, it has prioritized them over the lives of Yemeni people too.
The prime minister praised Saudi's "ambitious reform program" and offered the U.K.'s support "to progress and intensify these reforms, particularly on women's rights, and on universal human rights", the Downing Street statement said.