Agnes Uwimana, who has already served a one-year jail sentence for defamation and promoting division, was arrested on Thursday, Eric Kayiranga told AFP.
Kayiranga said Uwimana was accused of "incitement to civil disobedience, contempt of the head of state, spreading rumours to cause public disorder and denying the genocide of the Tutsis."
In recent weeks, the government media regulatory body had warned Uwimana, who runs the Umurabyo newspaper, over her articles. She resumed publication after serving her term in early 2008.
The New York-based Committee to Protect Journalists (CPJ) called for her immediate release. "Once again, Rwandan authorities invoke national security and the legacy of the 1994 genocide to silence one of the few dissenting voices in the shrinking independent Rwandan press," said CPJ Africa Advocacy Coordinator Mohamed Keita.
"We call on authorities to release Agnes Uwimana immediately; she should not go to prison for expressing her views a month before presidential elections," Keita said in a statement.
Also last Friday, the public prosecutor's office said it had concluded its probe and handed the courts a case of two people suspected of being behind the murder of a journalist last month.
Jean-Leonard Rugambage, who had accused the Rwandan government of being behind an assassination attempt on a dissident general in South Africa, was gunned down near his house in Kigali on June 25.
Kigali has denied any involvement in the Johannesburg shooting of General Faustin Nyamwasa.