Rwanda's president said the country had become "a family once again", while marking the 25th anniversary of the genocide that killed 800,000 people.
Paul Kagame, who led a rebel force that ended the slaughter, lit a remembrance flame in the capital Kigali.
Rwandans will mourn for 100 days, the time it took in 1994 for about a tenth of the country to be massacred.
Most of those who died were minority Tutsis and moderate Hutus, killed by ethnic Hutu extremists.
"In 1994, there was no hope, only darkness," Mr Kagame told a crowd gathered at the Kigali Genocide Memorial, where more than 250,000 victims are thought to be buried.
"Today, light radiates from this place. How did it happen? Rwanda became a family once again."
How is Rwanda remembering?
The commemoration activities began with the flame-lighting ceremony at the memorial. The flame will burn for 100 days.
The 61-year-old president, who has led the country since 2000, then delivered a speech at the Kigali Convention Centre.
He said the resilience and bravery of the genocide survivors represented the "Rwandan character in its purest form".
"The arms of our people, intertwined, constitute the pillars of our nation," he said. "We hold each other up. Our bodies and minds bear amputations and scars, but none of us is alone.
"Together, we have woven the tattered threads of our unity into a new tapestry."
He added: "The fighting spirit is alive in us. What happened here will never happen again."
Mr Kagame will later lead a vigil at the Amahoro National Stadium, which was used by United Nations officials to try to protect Tutsis during the killings.
Who is attending?
A number of foreign leaders are in the country for the events. They are mainly African, although Prime Minister Charles Michel represented the former colonial ruler, Belgium.
European Commission President Jean-Claude Juncker also attended.
French President Emmanuel Macron did not go, however. This week he appointed a panel of experts to investigate France's role in the genocide.
France was a close ally of the Hutu-led government prior to the massacres and has been accused of ignoring warning signs and training the militias who carried out the attacks.
France was represented by Herve Berville, a Rwandan-born MP.
Ugandan President Yoweri Museveni was also absent. He has been accused of backing Rwandan rebels who oppose Mr Kagame.
The vast number of people attending were ordinary Rwandans, including those who lived through the slaughter.
Olive Muhorakeye, 26, told Reuters: "Remembering is necessary because it's only thanks to looking back at what happened [can we] ensure that it never happens again."
BBC
