The service is being organised by Middle East Monitor as well as Al-Sharq Forum.
Multiple prominent journalists, politicians and activists are expected to take part in the memorial.
Some of the speakers include MP for Reigate Crispin Blunt, Middle East Guardian correspondent Martin Chulov, the Daily Telegraph’s former chief political columnist Peter Oborne and the executive director for Human Rights Watch Middle East and North Africa (MENA) Sarah Leah Whitson.
Trump invited Ms Cengiz to come to the White House earlier this month and vowed severe consequences against Saudi Arabia if they were behind the murder.
However, Ms Cengiz said she would only accept the invitation if he was willing to make a “genuine contribution” to the investigation.
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During an interview on Haberturk TV in Turkey on Saturday, she said that she was declining the invitation and accused the President of not being serious when it comes to investigating the killing.
In the interview, she said: “I demand that all those involved in this savagery from the highest to the lowest levels are punished and brought to justice.”
She added she had not been contacted by Saudi officials, but was still not going to attend the funeral in Saudi Arabia in the event that Mr Khashoggi’s body is found.
Following the disappearance f the Saudi journalist, Turkish authorities put Ms Cengiz under 24-hour protection over fears that she could be a target.
Mr Khashoggi disappeared at the beginning of this month after entering the Saudi consulate in Istanbul in order to et a document that proved that he was divorced.
After entering the consulate, he was tortured, killed and dismembered by a hit squad sent by the Saudi government according to Turkish officials.
For two weeks, Saudi Arabia constantly changed their story in regard to what happened to the journalist until they finally admitted that the killing was premeditated.
Since the killing, multiple countries, including Germany, have halted arms deals that they had with the Saudi government.
However, the UK has yet to make an announcement and Trump appears to be looking to continue a $110billion (£86.8billion) arms deal that the President claims will bring a million jobs to the US.
Actress Scarlet Johansen stated she was pulling out of a biopic film about Pulitzer Prize-winning photojournalist Lynsey Addario after she found out it was being part-funded by Saudi Crown Prince Mohammad bin Salman.