Gen Nkunda had been Rwanda's ally in eastern DR Congo.
Gen Laurent Nkunda, leader of the strongest rebel group in eastern Democratic Republic of Congo, has been arrested in Rwanda.
He crossed the border after resisting a joint Rwandan-Congolese operation to arrest him, both countries say.
Correspondents say it is a startling about-turn by Rwanda, which had been accused of backing Gen Nkunda.
BBC News website readers in the region have been sending their comments to the announcement:
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It's the best news I have had since the year started, cooperation and support for each other is what the great lakes region needs. We are tired of wars, we need development in the region. Room for reconciliation should prevail so as to live in harmony.
Daniel, Kampala, Uganda
Thank you God for the arrest of Nkunda, a ring leader of rebels groups. Rwanda and Congolese troops, viva for good job. The arrangement of prosecuting him lawfully must be express to ensure justice. Nkunda, we told you to stop atrocities and come to the table of negotiation peaceful but you ignored now face the door of justice.
Benson Nkya, Arusha, Tanzania
I'm so happy with this news, I think it is a lesson to even other rebels, like Joseph Kony in northern Uganda.
Mussa Loyde, Kibaha, Tanzania
Will the capture of Gen Nkunda by Rwandan forces prove to the world that Rwanda did not support him as has repeatedly been accused of? What pressure could have possibly forced Rwanda to turn against Nkunda! The world left Rwanda alone during the rainy days of the genocide, they should leave Rwanda alone as it rebuilds herself.
Nshuti Rugerinyange, Kigali, Rwanda
It is no secret that Nkunda was formerly a close ally of Rwanda's, and the fact that they have turned against him so quickly is unnerving. Previously I would not have been happy at these news, since Nkunda's forces were (although creating disorder) actually managing the North Kivu region well and protecting local Banyamulenge (Congolese Tutsis). However, since he had seemingly lost control over his troops I believe that he had become an obstacle to peace, and so I am glad that he has been arrested.
Anders Kirstein M., Kigali, Rwanda
That doesn't means that the Congolese are going to live in peace, never. Sorry for us Congolese, only God can help, no one else because of our stupid government.
Busisiri Bahati, Goma, RD Congo
It is the right thing to do.
Shudo Masawara, Kibati, RD Congo
Nkunda will face justice and possibly sent to the Hague for crimes and atrocities. Despite Ntangada's position today, he must be also arrested. If you shed blood, remember that that blood one day will start crying.
Ratshitunga, Lubumbashi, RD Congo
I'm very glad for this arrest. I think the troubles will end.
Allen Ingabire, Kigali, Rwanda
I hope this is the beginning of the pacification of the Great Lakes region. It's been long overdue and has all been a result of the indecisiveness of our leaders. He should face the law.
Byakagaba Micheal, Kampala, Uganda