The Rwandan president has said his nation bears the prime responsibility for the 1994 genocide but the outside world had stood by and let it happen.
Ceremonies began with a symbolic reburial of hundreds of victims
Paul Kagame was speaking to a crowd of 65,000 at the national football stadium marking the 10th anniversary under banners which read "Never again".
About 800,000 Tutsis and moderate Hutus were killed by Hutu militias after the assassination of an ethnic Hutu leader.
Mr Kagame again accused France of complicity in the genocide.
"They knowingly trained and armed the government soldiers and militias who were going to commit genocide and they knew they were going to commit genocide," he said.
Following this fierce criticism, French junior foreign minister Renaud Muselier decided to cut short his trip to Rwanda, the French news agency AFP reports.
Belgium, the colonial power, and South Africa have apologised for their inaction during the genocide.
Other Western countries have been criticised for not sending senior representatives to the ceremonies.
A national memorial was also opened on a hilltop in the capital Kigali, with the remains of hundreds of victims reburied in 20 coffins.
Women in traditional dress held up photos of their relatives killed in the 100 days of slaughter.
A museum on the site is to display pictures of the 300,000 children who are estimated to have been killed.
A monument to 10 massacred Belgian peacekeepers is also being inaugurated on Wednesday.
They were killed on 7 April 1994, trying to protect moderate Prime Minister Agathe Uwilingiyamana.
1994: RWANDA'S GENOCIDE
6 April: Rwandan Hutu President Habyarimana killed when plane shot down
April -July: An estimated 800,000 Tutsis and moderate Hutus killed
July: Tutsi-led rebel movement RPF captures Rwanda's capital Kigali
July: Two million Hutus flee to Zaire, now the DRC
UN Secretary General Kofi Annan called for a worldwide minute of silence at 1200 local time to mark International Day of Reflection on the Genocide in Rwanda.
Announcing an "action plan to prevent genocide", he said the world must be "better equipped to prevent genocide and act decisively to stop it when prevention fails".
Mr Annan was head of UN peacekeeping in 1994 and has come in for severe personal criticism. He is not attending the ceremonies.
Earlier, Rwanda's ambassador to Kenya Seth Kamanzi urged governments across the world to hand over fugitive genocide suspects to the International Criminal Tribunal for Rwanda.
"There can never be reconciliation as long as justice has not prevailed," Mr Kamanzi told a news conference in Nairobi.
About 15 people suspected of planning or organising the genocide are reportedly still at large.
Earlier this week, the former UN commander in Rwanda accused Western states of being responsible for the genocide.
Canadian General Romeo Dallaire said France, which led the small international peacekeeping force at the time of the genocide, the UK and the US in particular did not care enough to stop the killing.
"It's up to Rwanda not to let others forget they are criminally responsible for the genocide," he told a genocide conference in Kigali on his first visit to the country since 1994.
"There is no country today... which can wash its hands of Rwandan blood just by saying sorry."
The genocide was triggered by the shooting-down of ethnic Hutu President Juvenal Habyarimana's plane as it was coming in to land in Kigali.
The killing continued for 100 days before a Tutsi-dominated rebel army seized control.