Cengiz, who has learned English in order to campaign for justice for her murdered fiancée, said she never expected to have to address the British parliament on such grave and painful matters.
She said she wanted closure and this can only happen by getting truth and justice.
All G20 governments can make this happen, she said.
The UK parliamentary briefing was convened by Baroness Helena Kennedy QC, director of the International Bar Association's Human Rights Institute (IBAHRI).
"For all of us, it is our moral duty to solve this case," Cengiz said.
"Make sure that murderers and torturers pay the price for their inhumanity," she urged.
The UK should let its higher standards of fairness and honor shine through, to give the campaigners for justice some hope, she said.
The UK should use its authority to stop human rights abuses, she said, and not sell arms which will be used to harm civilians and crush dissent.
The Saudi Crown Prince was rightly stopped from buying a football club in the UK and he should be banned from coming to Britain altogether, she said.
The British Government should not deal with the Crown Prince as a friend and ally, she urged.
“Don’t let him seduce you with his trade and finances. The truth is that he is a murderer and torturer,” she said.
“If you befriend him, you become complicit in his dirty business. I ask you to listen seriously to my appeals to you.
“You have a chance to make a real difference in the world, to do what is right to bring justice to this case,” she said.
Lawyer for Khasoggi’s grieving fiancée, Rodney Dixon QC, told the briefing that truly barbaric acts are not crimes that can be limited by borders and labelled ‘internal matters’.
World leaders at G20 summits should know better than anyone the international law, he said.
Saudi Arabia is wrong to assert that its national trial has settled this case, he said.
Those proceedings were hopelessly inadequate and conducted entirely in secret, the barrister said.
“We will continue to pursue the need for an independent and impartial criminal trial to hold those responsible to account, whether through specialised international court, or though civil actions, as has been launched in the US, which can serve valuable purpose in revealing truth about what happened,” he continued.
Dixon said the bubble has burst on the prosecution and arrest of world leaders who believe they can get away with crimes.
The crimes against Khasoggi are international and have truly shocked all nations, and require prosecution at the highest level, he said.
Moderator Baroness Helena Kennedy QC said Rodney Dixon was an outstanding human rights lawyer.
Documentary filmmaker Brian Fogel has made a film The Dissident on the untold story of the murder of Jamal Khashoggi.
Fogel said there had been complicity among G20 leaders to give those responsible a 'slap on the wrist'.
He said he hoped his film would be a rallying call to populations led by G20 leaders, to reconsider their business ties to states that commit murders.
Baroness Kennedy said she paid tribute to Brian for visualizing and bringing to a wider audience the horror of the Khashoggi murder.
Human rights campaigner Thor Halvorssen said it was important to humanise the victim of this terrible murder, and the film-maker has done a great job at this.
The Human Rights Foundation will campaign to bring to light the pressure exerted on western companies such as Netflix and Amazon, which have allegedly bowed to business pressure not to show the film, he said.
It should be a source of shame that Saudi is criminalizing women for asking for basic human rights, and yearning for fairness and justice, Baroness Kennedy said, which undermines any claim that that country is coming into modernity.