Gennady Gudkov, deputy head of the Just Russia party, (R) gestures while leaving Russia's lower house of parliament, the State Duma, in Moscow, on Sept. 14, 2012. (AFP/Getty Images)

Russia’s parliament voted to strip an opposition leader of his seat Friday, leaving him vulnerable to criminal prosecution in what his supporters describe as another step toward silencing criticism of President Vladimir Putin.

Gennady Gudkov, a 56-year-old former KGB officer who emerged in recent months as an advocate for the rule of law and freedom of speech, was accused of illegally running a business while serving in the State Duma. He called the charge politically motivated.

The expulsion of Gudkov, a lonely voice of criticism in the legislature, fit a pattern of measures apparently aimed at curtailing protest following Putin’s return to the presidency in May after four years as prime minister.

“This is a political revenge, extrajudicial reprisal,” Gudkov told the Duma, “carried out upon command from the Kremlin.”

Gudkov is a member of the Just Russia party, which was formed in 2006 as an ostensible alternative to Putin’s United Russia party. Serious opposition parties have been kept out of parliament through the manipulation of election laws, and until recently Just Russia operated more as a Kremlin ally than an opponent. But last winter, as demonstrators took to the streets to protest vote-rigging and limits on democracy, Gudkov unexpectedly took up their cause.

Those protests, the first serious demonstrations against Putin since he took power in 2000, clearly rattled him. Putin first blamed the United States, claiming it had financed the marchers, who he said were called out by a signal from Secretary of State Hillary Rodham Clinton.

If the protests angered Putin, they awakened something in Gudkov. He began speaking at protest rallies, embracing free elections and more robust democracy.

As Putin’s critics saw it, even the rare public gadfly was one too many. Gudkov soon came under investigation. In July he was forced to sell his security company, which he started before he entered the Duma in 2001. Then prosecutors asked the Duma to strip him of his seat and parliamentary immunity so he could be prosecuted.

Friday’s vote to remove him was 291 to 150, with three abstentions. Gudkov became only the second lawmaker expelled by a Duma vote. The first was Sergei Mavrodi, the operator of a financial pyramid scheme who ran for office to win immunity and was removed in 1996.

“What is going on now is a disgrace to the country,” Gudkov said in an emotional speech to the Duma before the vote. “Instead of taking the matter to court, even our court, you have decided to quickly get rid of your political opponents by voting.”

Gudkov accused legislators of “trying to use repressions to shut up the people.” He warned, “No, you won’t be able to push the people back into their kitchens — people will continue to go out and demand fair life and fair elections.”

Friday’s vote came on the eve of an opposition rally, raising suspicion among organizers that authorities intend to arrest Gudkov along with other protest leaders at Saturday’s march, the first major rally since June.

Seventeen activists are under investigation for suspicion of inciting mass rioting after a clash between police and demonstrators in May, and 12 of them remain in pretrial detention.

During the summer, the Duma passed a series of laws aimed at chastening dissidents. One drastically raised fines for violations of the protest law. Gudkov — along with two other Just Russia members, his son Dmitri and Ilya Ponomaryov — organized an 11-hour filibuster against it. The effort failed, but it astonished many voters unaccustomed to real debate in the Duma, and it may have been the act of defiance that doomed Gudkov.

Other laws followed. One requires nongovernmental organizations involved in civic affairs to register as foreign agents if they receive funds from abroad.

In addition, anti-corruption crusader Alexei Navalny has been charged with embezzlement. Three members of a punk rock band were sentenced in August to two years in jail for an anti-Putin protest in a cathedral. The wife of a leader of a radical opposition group was sentenced to eight years — prosecutors had asked for only four — on what supporters called a fabricated drug charge.

This week, the European Parliament passed a resolution expressing concern that Russia was using the legal system to restrict freedom of speech and assembly.

Sergei Mitrokhin, leader of the liberal Yabloko party, said at least 21 other Duma deputies own businesses, but none of them has been warned of illegal activity. Gudkov’s expulsion, he said, was not only politically motivated but illegal.

Gudkov told the Duma he was embarrassed for the country.

“But I want to leave in order to come back and to build a new country, which will make its people proud,” he said. “We will win, the truth will win, the constitution will win, and we’ll come back, believe me.”