NAIROBI, Kenya — Rwandan authorities arrested a top opposition politician on Wednesday and charged her with genocide ideology — a contentious crime that many critics say has been used to stifle dissent — and with cooperating with a notorious rebel group.

The politician, Victoire Ingabire, has been one of the most vocal opposition figures in a country that seems to have grown increasingly intolerant of political challenges. She was summoned to a police station at 9 a.m., according to her assistant. Mrs. Ingabire, an accountant who had lived in Europe for years, has been under investigation since nearly the moment she returned to Rwanda in January and announced she was running for president.

Investigators have interrogated her several times on suspicion of instigating ethnic divisions. She said in an interview last month that she was being persecuted for simply challenging the government.

Rwandan officials have denied that they are cracking down on political opposition, and they point to the impressive strides the economy has made in the past 10 years as evidence that their leadership is improving the country. Rwanda is scheduled to hold a national election in August, and many analysts predict more turbulence — and more arrests — in the months to come.