An international conference in Kigali also opened on Sunday afternoon, after participants had been shuttled to areas outside the Rwandan capital where some of the victims - a total of at least 800,000 and up to one million - were killed in 100 days in 1994.
At the churches in Nyamata and Ntarama, south of Kigali, about 15,000 people were massacred after they sought refuge there.
The cemetery of Nyanza, another stop on the tour on the outskirts of the capital, is the burial ground for 3500 people who were abandoned in a schoolyard by UN peacekeepers, and later massacred.
Hours later, the three-day conference, focused on the hopeful theme of "preventing and banishing genocide forever", opened with a condemnation of international inaction.
"The international community could have kept the genocide from taking place," Francois-Xavier Ngarambe, the president of an association of genocide survivors, told the gathering. The victims, he added, were "a people who meant nothing to the interests of the great powers".
World leaders initially downplayed the 1994 killing of nearly 800,000 Tutsis and Hutu moderates by Hutu extremists who mostly hacked and bludgeoned their victims to death with machetes, spiked clubs and garden tools.
President Paul Kagame, who headed the Tutsi rebel army that in 1994 ousted the extremist Hutu regime and ended the genocide, addressed the participants.
He said he was so frustrated by world inaction during the country's 1994 genocide that he considered attacking the local UN mission and stealing its weapons to stop the mass slaughter of civilians.
Paul Kagame led the Tutsi army
that ousted the genocidal regime
The overwhelmed UN mission in Rwanda, led by Canadian Lieutenant-General Romeo Dallaire, did not have the mandate to intervene even as Rwandans were being butchered at a rate of 8000 people a day.
"Dallaire had soldiers, weapons and armoured personnel carriers and I confess for the first time that I contemplated taking those arms from him by force," Kagame said, drawing gasps and some applause from hundreds of government officials and diplomats.
"I rolled it in my mind and talked it over with my colleagues in the bush, but we knew that it would open another front for us to fight, so after second thoughts, I abandoned the idea," Kagame told the crowd.
Dallaire, who returned to Rwanda on Friday for the first time since 1994, is widely regarded as a hero who repeatedly tried in vain to alert the world to Rwanda's plight.
The Rwandan genocide was sparked by the assassination of the Hutu president, Juvenal Habyarimana, on 6 April 1994, whose plane was shot down as it came in to land at Kigali airport.
In a deliberate campaign of bloodletting, ethnic Tutsis and politically moderate Hutus were targeted from the following day, and only in July 1994 did a mainly Tutsi rebel movement led by Kagame seize Kigali and put an end to the carnage.
A 1999 report laid the blame on UN Secretary-General Kofi Annan and other UN officials, as well as on the United States and other Security Council nations, for failing to take action to stop the killing.
Various world leaders have expressed regret about not having done more to prevent genocide in the tiny central African country 10 years ago, but Kagame's comments underscored the sense of abandonment still felt by many Rwandans.
Annan, who was head of peacekeeping operations at the world body during the 1994 massacres, recently accepted institutional and personal blame for not doing more to prevent Rwanda's 100 days of slaughter.