DAKAR, Senegal — The former president of Chad, Hissène Habré, who is accused of the killing and torture of thousands of opponents, was arrested here on Sunday by the Senegalese police, said the organization Human Rights Watch.

Victims of Mr. Habré’s brutal eight-year reign in the 1980s have been fighting for more than two decades to bring him to justice, and Sunday’s arrest was hailed as a decisive step by human rights advocates.

Mr. Habré fled into exile here after being overthrown in a coup in 1990 by Chad’s current strongman, Idriss Déby. Ever since, Senegalese governments have stalled and stymied the efforts of his victims to have him prosecuted. Finally, after the election of a new president in Senegal last year, a special court was established to try Mr. Habré in Dakar, the capital.