A leading human rights activist, Lev Ponomaryov, said Monday that he had been charged with slander for calling the country's top prison official "the author of a sadistic system of torture."

Moscow prosecutors visited Ponomaryov at his home Thursday and initially questioned him "as a witness," Ponomaryov said.

"It soon became clear that I would turn into a suspect," he said.

Ponomaryov said he was charged with falsely accusing a civil servant of committing a serious crime, which carries a maximum sentence of three years in prison. The prosecutors also asked Ponomaryov to sign a document agreeing not to leave Moscow for the duration of the investigation, which he did.

The case dates back to a 2006 interview that Ponomaryov gave to the Regnum news agency. In the interview, Ponomaryov called Federal Prison Service chief Yury Kalinin "the author of a sadistic system of torture" and said he was responsible for a network of 40 prisons that were effectively "torture zones."

Kalinin filed a complaint, and in April Moscow's Presnensky District Court ordered Regnum to publish a correction, which it did in October.

With the case apparently over, it was unclear as to why the prosecutors had charged Ponomaryov. Calls to prosecutors and the Federal Prison Service went unanswered Monday, a public holiday.

The country's Kremlin-nominated human rights ombudsman, Vladimir Lukin, also called the conditions in many prisons "close to torture" in a report published earlier this month.

Ponomaryov said the charge against him was fabricated. "Kalinin is eager to show the new president that he is indispensable to the regime," he said.

Dmitry Medvedev, President Vladimir Putin's handpicked successor, is expected easily to win Sunday's presidential election.

Ponomaryov, 67, is one of Russia's most prominent activists and is especially vocal on the treatment of prisoners. A former State Duma deputy, he is the executive director of the For Human Rights group and a member of the Moscow Helsinki Group. Ponomaryov has attended many opposition rallies and has been detained numerous times. He said he intended to participate in a postelection Dissenters' March on Monday.

Ponomaryov had a run-in with authorities in June, when he was questioned by Federal Security Service officials over a speech he made at a January 2007 rally in defense of two businesspeople accused of illegal trafficking of ethyl ether.

Putin in 2006 signed a law allowing the slander and libel of government officials to be classified as extremism. A series of cases has followed.

A Perm reporter was questioned last week and may face charges after he wrote an article identifying what he characterized as positive similarities between Putin and Adolf Hitler.

In September, Saratov prosecutors charged Sergei Mikhailov with extremism after his newspaper, The Saratov Reporter, published a photo portraying Putin as the popular fictional spy Otto von Stirlitz. The charges were dropped earlier this month.

Ivanovo journalist Vladimir Rakhmankov was convicted in October 2006 of publicly insulting a public official and fined 20,000 rubles ($750) for referring to Putin as "a phallic symbol."