RIGA, Latvia - Russia's Federal Security Service on Tuesday arrested a renowned left-wing sociologist and former Soviet dissident, Boris Kagarlitsky, on a charge of "justifying terrorism" over comments he posted in October on social media - the latest move in a widening wartime crackdown on dissent by President Vladimir Putin's government.

Kagarlitsky, a well-known Marxist thinker and prolific author and activist, was transferred by the authorities to the northern city of Syktyvkar, more than 800 miles from his Moscow home.

Despite thousands of recent arrests of rights activists, politicians and journalists, Kagarlitsky's arrest shocked Russia's intelligentsia and was condemned by prominent pro-Kremlin analyst Sergei Markov, who called it a "gross political mistake" in a post on Telegram - a rare criticism from the pro-war camp about the arrest of an antiwar figure.

"Stay away from Kagarlitsky!" Markov said, saying that imprisoning him would cause "huge harm to Russia in the world."

The arrest highlighted the deepening fractures across Russian society as Putin persists with his vicious war in Ukraine despite signs that his military is largely stalled and slowly losing ground amid a grinding Ukrainian counteroffensive.

On Wednesday, Russia's Defense Ministry claimed to have defeated a major Ukrainian attack near Orikhiv in Zaporizhzhia, in eastern Ukraine, declaring it had destroyed 20 Ukrainian tanks and 10 armed personnel carriers and killed 100 Ukrainian soldiers.

But a pro-war military blog, Zapisky Veterana, which means "Notes of a Veteran," reported more modest Ukrainian losses of "several destroyed enemy vehicles and more than a dozen prisoners."

The Ukrainian Defense Ministry, meanwhile, claimed to have blunted Russian attacks on Wednesday at numerous locations along the 600-mile-long front and to have made some gains in retaking occupied territory in the Zaporizhzhia region, as part of "an offensive operation in the Melitopol and Berdyansk directions."

Kyiv said its advancing forces were "entrenched at the reached boundaries" and inflicting "fire damage."

Heavy fighting and artillery and airstrikes were reported throughout the day, including in the east near Bakhmut and Avdiivka. On Wednesday evening, Moscow unleashed another barrage of missile strikes on targets across Ukraine, including an airfield in the Khmelnytskyi region.

In a statement posted on social media, Ukraine's Air Force said that Russian forces fired 39 cruise missiles over the course of the day, of which 36 were shot down by Ukrainian air defenses.

"The missiles entered the airspace of Ukraine from the south-eastern direction, went to the West, constantly changing the direction of flight," the statement said.

However, four Kinzhal hypersonic ballistic were also launched, about which the Air Force said it was "clarifying" details.

Publicly, Putin has been bristling with confidence that Ukraine's counteroffensive has "failed" and that the West's cautiously calibrated weapons supplies will not help Ukraine win the war, weeks after a NATO leaders' summit in Vilnius, Lithuania, cheered Moscow by failing to spell out a clear path for Ukraine to join the alliance.

But there were also new signs Wednesday that Russia's military continues to face personnel challenges, as Russian lawmakers passed a law to expand the compulsory conscription pool.

The conscription parameters, almost certain to be signed into law, alarmed many, because they ditch an earlier plan to raise the lowest age that men can be conscripted from 18 to 21, and the upper limit from 27 to 30. Instead, the Russian parliament, a rubber stamp for the Putin regime, moved only the upper limit, widening the range by three years, to 18 to 30.

"The State Duma smells 'a big war,'" ran the headline in the Nezavisimaya Gazeta. Influential lawmaker Andrei Kartapolov, head of the defense committee of the State Duma, or lower house of parliament, said the law to widen the conscription age was drafted "for a big war, for a general mobilization."

"And now it already smells like the biggest war," Kartapolov continued, according to the newspaper, a sign that Putin's regime is preparing to fight an extended, ruthless, bloody war for as long as necessary, in the conviction that Western military support will dissipate because of electoral pressures on Western politicians.

Kartapolov warned that it was inappropriate for society to debate the conscription bill because it was too sensitive, underscoring official wariness of about public opposition to mobilization and conscription.

In an unusual show of dissent among Russia's usually tame lawmakers, several expressed opposition to the plan to depart from the earlier planned law. Influential senator Viktor Bondarev, head of the defense committee in the Federation Council, the upper chamber of parliament, said he was bewildered by the abrupt change, Nezavisimaya Gazeta reported, and added that the revision had never been discussed with the upper house.

Another prominent senator, Andrei Klishas, head of the Federation Council legal committee, strongly backed Bondarev, in a sign of the disquiet over the secrecy and haste surrounding the bill.

The arrest of Kagarlitsky, the left-wing sociologist, occurred just days after Russian authorities arrested Igor Girkin, a prominent nationalist war blogger, on charges of promoting extremism. Girkin was the first pro-war figure who had expressed harsh criticism of Putin and Russian military leaders to be arrested under draconian new laws barring criticism of the military.

Kagarlitsky, a prominent critic of the war in Ukraine, was a dissident in the Soviet era and jailed for "anti-Soviet propaganda" for more than a year in Lefortovo prison during the leadership of Leonid Brezhnev. He founded a left-wing Moscow think tank, the Institute for Globalization Studies and Social Movements, in 2007. His institute was declared a "foreign agent" by Russia's Justice Ministry in 2018, and Kagarlitsky was personally branded with the label in May 2022.

"Boris Kagarlitsky today is probably the most influential Russian politician and expert of the left camp in the world," Markov said, calling on the Kremlin to "actively work with him," not jail him.

Kagarlitsky was arrested after the head of the Federal Security Service (FSB) in the northern Komi Republic determined that an item posted on social media constituted justification of terrorism. The FSB initiated charges, leading to Kagarlitsky's removal to the remote city of Syktyvkar.

Kagarlitsky's lawyer, Sergei Yerokhov, told Russian media outlet Agentstvo that he was charged over a post he made following the October attack on the Crimean Bridge. The comment, which appeared to have been carefully worded to avoid breaching Russia's strict laws banning criticism of the war, said that the attack on the bridge would create military supply issues.

"From the moment of its grand opening, the Crimean Bridge has been not only a strategic, but also a symbolic object, the main achievement of the Putin era and material proof that in our country, even against the backdrop of total theft and inefficiency, it is still possible to achieve practical results if you invest some 400 to 500 percent of technologically necessary funds," Kagarlitsky's post said.

Kagarlitsky appeared in court Wednesday and was ordered to be held in a pretrial detention center for two months, pending trial, although in Russia such detentions are usually extended.

Kagarlitsky said the decision to charge him was political. "Why, by whom and how - I can't say," he said, adding: "I categorically disagree with the charges I am facing and believe that this is a political decision. The statement I am accused of dates back to October last year. That is, it took 10 months to incriminate me, although it was all over the internet."

Kagarlitsky has often argued that Russians, alienated by the war, disillusioned with corruption and barred from expressing their viewpoints, instead have retreated into their own personal worlds, avoiding politics, protests and talk about the invasion.

The process of his arrest was unusual, because prosecutions normally occur in the city where defendants reside, and it raised the specter that any prominent figure could face arrest if an FSB regional boss in a far-flung outpost decides to act over comments on social media.

The proposed changes to the conscription law suggest Russia is continuing to seek reinforcements to send to Ukraine even after an aggressive recruiting campaign in prisons, which allowed convicts to earn a pardon in exchange for enlisting to fight.

In recent months, Russia has quietly been running an intensive military recruitment program to boost its forces fighting in Ukraine, and has taken other measures to expand the pool of men who can be conscripted.

On Monday, Putin signed a law increasing the upper age limit that military reservists can be mobilized from 50 to 55. For officers, it increased from 55 to 60. Average life expectancy for Russian men is 67.

In yet another planned measure to free up Russian forces to fight in Ukraine, a separate law would allow the formation of people's militias, which would enable Russian governors to form and arm their own forces, "to strengthen the protection of public order and ensure public safety during the period of mobilization, during martial law, in wartime."

The Washington Post's Natalia Abbakumova in Riga contributed to this report.