Victoire Ingabire Umuhoza was to challenge president Paul Kagame in August’s election.
However, a police spokesman said she had committed serious crimes.
She has previously said she had been harassed since returning from exile in January. One of her aides was jailed last month.
Her party, the Unified Democratic Forces, called her arrest unlawful and demanded her immediate release.
The BBC’s Geoffrey Mutagoma in the capital, Kigali, said Ingabire’s home had been surrounded by heavily armed security forces.
The US-based Human Rights Watch says Rwanda’s government is attacking and intimidating its critics in the run-up to August’s presidential election.
But Kagame has said in the past he respects people’s rights but will not tolerate anyone undermining peace and stability. –– BBC.