The police in Tunisia on Tuesday arrested Ayachi Hammami, a prominent opposition figure and human rights lawyer, according to Human Rights Watch, the latest sign of backsliding toward authoritarianism in the country that ignited the Arab Spring.

Mr. Hammami, who was arrested at his home outside the capital, Tunis, was sentenced by an appeals court last week to five years in prison for belonging to a terrorist group and conspiring against state security, according to rights groups. His sentence was part of a broader case against roughly 40 people who were convicted on conspiracy charges.

Mr. Hammami and the other defendants, including opposition leaders, lawyers, businessmen, rights activists and journalists, were originally sentenced in April, according to Human Rights Watch. Their subsequent appeal failed last week, when a judge sentenced them to prison terms of up to 45 years.

Rights groups have said that the charges were baseless and politically motivated.

“This isn’t justice — it’s a sham trial, plain and simple,” said Ahmed Benchemsi, a spokesman for Human Rights Watch. “Hammami wasn’t punished for any crime, but for standing up against oppression and defending human rights.”

After Mr. Hammami was arrested, a prerecorded video appeared on his Facebook account in which he condemned his imprisonment and called it a “political decision” by Tunisia’s president, Kais Saied. Mr. Hammami also said he was embarking on a hunger strike in protest.

“Just as I fought outside prison for democracy, human rights, freedoms and social justice, I will turn the cell in which Kais Saied imprisons me into a space of struggle as well,” he said.

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