Hatice Cengiz, fiancee of the murdered journalist Jamal Khashoggi, has said Saudi authorities must not be allowed to cover up his killing, and called on western countries to demand justice.

Speaking at a memorial event in London on Monday, she said through a translator: “I believe that the Saudi regime knows where his body is. They should answer my demand, for this is not only the demand of a fiancee, but a human and Islamic demand, from everyone, every nation.”

Cengiz criticised the response to Khashoggi’s death from some countries and singled out Donald Trump for criticism, saying she feared the US president was “paving the way for a cover-up”. “I am disappointed by the actions of the leadership in many countries, particularly in the US,” she said.

“President Trump should help reveal the truth and ensure justice be served. He should not pave the way for a cover-up of my fiance’s murder. Let’s not let money taint our conscience and compromise our values.”

Jamal Khashoggi killing: what we know and what will happen next Read more

Trump has given mixed messages over the case, vowing a severe response but also saying that the United States wanted to preserve its close relationship with Saudi Arabia.

Cengiz – who accompanied Khashoggi to the Saudi in Istanbul consulate on 2 October and raised the alarm when he failed to re-emerge – said her life had been turned upside down on that “horrible day”.

She added: “If only I had known what would happen, I would have entered the consulate myself and stood in front of the murderous crew. If only I had known … I would have done everything to stop him entering that building. We never imagined such a level of barbarity cruelty and evil.”

She described her fiance as a patriot who felt it was his duty to become “the voice of the voiceless”. “He wrote what he believed and only wrote what he knew was right, and not what would satisfy the powerful,” she said.

Speaking at the same event, the former chairman of the Commons foreign affairs select committee, Crispin Blunt, said Khashoggi’s death represented a moment of truth. He said: “Britain should consider every possible measure to mark Jamal’s murder. He called for the release of women’s rights campaigners, an end to the death penalty, the end of the criminalisation of satire, and the definition of dissent as terrorism.”