London (CNN) Hatice Cengiz, the fiancée of Jamal Khashoggi, has called upon US President Donald Trump to "help reveal the truth" about the Saudi journalist's killing.

"I am deeply grateful for the solidarity of people all over the world. I am, however, disappointed in the actions of the leadership in many countries, particularly in the US," Cengiz said Monday night at a memorial in London for the slain Washington Post columnist.

"President Trump should help reveal the truth and ensure justice be served. He should not pave the way for a cover-up of my fiancé's murder. Let's not let money taint our conscience and compromise our values," she added.

Her remarks came days after she rejected an invitation from Trump to visit the White House.

Cengiz described Khashoggi as a "martyr for a cause" and said she hoped his body would be returned soon so his loved ones could bury him.

"He is a martyr for the struggle for democracy and freedom in our part of the world. I want to bury the body of my beloved Jamal -- my martyr -- surrounded by the prayers of his friends and loved ones. I want to know: Where is his body?" she said.

"I believe that the Saudi regime knows where his body is ... I want justice to be served. Not only for those who murdered my beloved Jamal, but for those who organized it and gave the order for it."

Saudi Journalist Jamal Khashoggi with his fiancée, Hatice Cengiz.

Cengiz added that she wanted "the role of the political leadership in this brutal killing to be brought to light" and asked the global community to assist in holding "the perpetrators and their masters to account."

"There should be no cover-up. Jamal was my beloved fiancee, but he was also a gentle human being, a loving man, a journalist and a true believer in democracy and freedom in the Arab world," she said.

"Let's demand justice for Jamal and stand up for his ideals," Cengiz concluded before receiving a standing ovation.

Changing stories on Khashoggi's death

Khashoggi, 59, was last seen alive on October 2 entering Saudi Arabia's Istanbul consulate around lunchtime, where he had hoped to obtain paperwork that would have allowed him to marry Cengiz, a Turkish national. When he failed to re-emerge, his fiancée -- who had been waiting outside -- alerted Turkish authorities just before 5 p.m. that something was wrong.

CCTV image of Khashoggi entering the Saudi consulate in Istanbul on October 2.

In the month since, Saudi officials have released several shifting accounts of the events surrounding Khashoggi's death.

Initially, the kingdom said the journalist had left the consulate alive shortly after arriving. It later said he had died in a fistfight when a discussion turned violent. Last Thursday, the country's attorney general said new information received from Turkish investigators now led them to believe the killing was premeditated

The killing of Khashoggi, a prominent Saudi journalist and critic of the regime, has sparked international condemnation and outrage. Riyadh has maintained that neither King Salman nor Crown Prince Mohammed bin Salman knew of the operation to target Khashoggi.

US officials have said such a mission -- including sending 15 men from Riyadh -- could not have been carried out without the authorization of bin Salman, the country's de facto ruler.

Khashoggi's friends pay tribute

Many of Khashoggi's friends and loved ones gathered in London on Monday to remember his life and celebrate his work.

Wadah Khanfar, a former director general of Al Jazeera, said he had spoken to Khashoggi in the days before his killing when they discussed the journalist's family, his hopes for the future and his upcoming nuptials. Khanfar said he and many others joined the family in their grief and would continue to follow the principals Khashoggi stood for.

"Some people, you don't have to build gilded statues and marble monuments for them to be remembered. ... We might not have a grave for the body of Jamal but I think in every heart, in every good heart on this planet, Jamal shall reside and his memory shall continue," Khanfar said in a passionate speech.

Sarah Leah Whitson, executive director of Human Rights Watch's Middle East and North Africa Division, read a letter she'd written to Khashoggi.

"I don't believe in the afterlife, but I want to believe you can hear me. I know I can hear you. ... Your voice is only amplified in your death. A roar to your real life whisper reaching millions more than you might have ever dreamed of reaching. Two million Twitter followers? Ha! That's nothing to the audience you have commanded and raised up today," Whitson said.

"I will honor your commitment to standing by Saudi activists, writers and scholars who today are languishing in jail but will one day lead their people forward," she said. "Generations of Saudi children will remember you and esteem you as the Saudi man who did not bow down, the Saudi man who raised his voice, the Saudi man who took a detour for love, the Saudi man who paid for his freedom with his life. You have given your people a hero to believe in."

Saudi Arabia has detained 18 people and dismissed five high-ranking officials , including the crown prince's media chief and the deputy head of the Saudi intelligence service, as it continues its investigation into the killing of Khashoggi.

On Friday, Turkish President Recep Tayyip Erdogan demanded the Saudis hand over the suspects , but it appeared an extradition request is likely to be denied.

"The individuals are Saudi nationals," Saudi Foreign Minister Adel Al-Jubeir said on Saturday. "They're detained in Saudi Arabia, and the investigation is in Saudi Arabia, and they will be prosecuted in Saudi Arabia."