Rwandan police arrested the country's most prominent opposition leader Thursday and accused her of forming a terrorist organization, months after she was barred from challenging the president in an election.

Authorities said Victoire Ingabire was implicated during investigations into the activities of a man who was a former commander of a Hutu militia group operating in neighboring Congo.

Ingabire already faced charges of genocide ideology after she stated publicly that crimes committed during the 1994 Rwandan genocide against Hutu citizens should be investigated....