The Washington Post columnist was killed inside Saudi Arabia's Consulate in Istanbul previous year.
Canadian Minister of Foreign Affairs Chrystia Freeland and UK Foreign Secretary Jeremy Hunt are co-hosting the conference.
Human rights lawyer Amal Clooney, who was appointed a special envoy for media freedom by Hunt, will also convene a panel of legal experts who will advise governments on how their laws or actions affect media freedom.
The pair were speaking at the Global Conference for Media Freedom, an event organised by the Foreign Office in coordination with the Canadian government.
The Committee to Protect Journalists (CPJ) on Wednesday said it was concerned that the British government had denied accreditation to Russian journalists who wanted to attend the global conference on media freedom taking place in London.
Ms Callamard's 101-page report, published in June, said there was "credible evidence" that Saudi Arabia's Crown Prince Mohammed bin Salman and other high-level officials were individually liable for the killing of Jamal Khashoggi. According to the United Nations cultural body Unesco 99 media workers were killed a year ago.
The gathering announced the founding of a global fund to provide training, legal support and other resources for journalists in danger zones, administered by the United Nations cultural body.
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The British government said Russian news agency Sputnik and state-owned TV network RT were excluded due to their alleged "active role in spreading disinformation". At its best, a free media both protects society from the abuse of power and helps to release the full potential of a country.
And today we send a strong message that media freedom is not a Western but a universal value....
Asked about US President Donald Trump's attacks on the media for "fake news", Hunt said: "I wouldn't agree with it".
In his speech, Mr Hunt said: "Powerful people value their reputations, so the sunlight of transparency is the greatest deterrent to wrongdoing".
Hunt also defended the publication of the private diplomatic cables highly critical of Trump, sent by the United Kingdom ambassador to the US, Kim Darroch.
Clooney also took aim at Trump, saying "the country of James Madison" - one of America's founding fathers and a champion of a free press - "has a leader today who vilifies the media".
"And I think we have to remember whatever the political battles we face in countries where we are used to press freedom and it's part of our daily lives and we've never known anything else, we have to remember that what we say can have an impact in other countries where they can't take press freedom for granted".
Mr Hunt also said he disagreed with Mr Trump's use of language towards journalists.