Kenyan President Mwai Kibaki and President Idriss Déby Itno of Chad jetted in before lunch time.
Incidentally, the two leaders are under fire from the International Criminal Court (ICC) and western government for having hosted the indicted Sudanese leader Omar El Bashir instead of arresting him.
No government delegation from Sudan is expected at the event.
President François Bozize of Central African Republic, Togolese president Faure Gnassingbé and President Yayi Boni of Benin arrived in the country on Saturday.
Government says some 16 Heads of State have confirmed their attendance – with Uganda and Tanzania sending their Prime Ministers.
On Sunday, the leaders will meet to discuss Africa’s progress towards the Millennium Development Goals. President Kagame was named in June by UN Secretary General to co-chair with Spanish PM Jose Luis Rodriguez Zapatero, a high-profile global team on the MDGs.
In July, the team met in Spain for the first meeting.
The leaders in Kigali will share their country’s experiences ahead of the 65th session of the United Nations General Assembly to be held this month in New York whose theme will be on MDGs.
Other leaders expected on Sunday include Nigerian leader Goodluck Jonathan and Blaise Compaoré of Burkina Faso.
Other leaders will be arriving Monday morning ahead of the grande fete.