MOSCOW—Challenging Vladimir Putin has always been difficult. Now his opponents say it is almost impossible, as the Kremlin launches a full-court press against anyone daring to stand up against the Russian president.

In recent weeks, authorities have stepped up a campaign to suppress critics and organizations seen as being antigovernment ahead of parliamentary elections in the fall. Opposition leaders have been detained, human rights and legal groups forced to disband and independent media organizations sidelined on an almost weekly basis since the April jailing of Alexei Navalny, Russia’s best-known dissident, who earlier survived a nerve agent attack.

For some political commentators and Kremlin-watchers, the clampdown is further evidence that Mr. Putin’s regime is growing stronger, despite social discontent over the economy and Western sanctions for its alleged transgressions, including human-rights abuses.

Russian President Vladimir Putin’s supporters are set to dominate in September’s parliamentary elections.

Russian President Vladimir Putin’s supporters are set to dominate in September’s parliamentary elections.

Photo: Alexei Nikolsky/TASS/Zuma Press

He doesn’t appear to feel threatened by the opposition or pressure from the U.S. and cares little about appeasing the public, analysts said. Authorities instead appear willing to use a range of tools to quash any space for organizing street protests or investigating the financial affairs of Mr. Putin and his inner circle, they said.

“It shows that Putin is now very confident in himself and he can rely on force, believing that it isn’t necessary to make some kind of trade-off between the society and the government,” said Vladislav Inozemtsev, director of the Moscow-based Center for Post-Industrial Studies, a nonprofit think tank.

The latest move saw Russian authorities block access to Mr. Navalny’s website, Navalny.com, and the sites of dozens of others on Monday, including those of his anticorruption fund and individuals and organizations connected to his cause.

The site had been one of Mr. Navalny’s primary ways of staying in touch with supporters after his imprisonment for violating the terms of his parole for a previous conviction that he says was politically motivated. The move to block it comes after his movement was designated an extremist organization, putting it on a par with terrorist groups such as al Qaeda or violent religious cults. That followed a dramatic incident last year when Mr. Navalny narrowly survived an alleged assassination attempt after he took ill on a flight from Siberia to Moscow. His supporters whisked him away for treatment in Berlin, where doctors and Western intelligence agencies determined that he had been poisoned by Novichok, a Soviet-era nerve agent.

Mr. Navalny and his supporters have since focused on September’s parliamentary elections as a way to deflect support from the ruling United Russia party by exposing government corruption and boosting Mr. Putin’s opponents. In all, 450 seats will be contested.

But the past three months have seen a sharp escalation in the way authorities have used draconian laws to brand opposition groups and individuals as “foreign agents” or “undesirables,” compelling at least 17 media organizations and journalists to cease operations or limit the scope of their work since late April.

Among them, Open Russia, a pro-democracy organization founded by exiled business tycoon Mikhail Khodorkovsky, said in May it would halt its activities in Russia to avoid potential criminal prosecution of its staff. The London-based group was labeled as undesirable back in 2017, but new laws targeting anyone cooperating with such groups have raised the stakes.

Andrei Pivovarov, chief executive of Open Russia, a pro-democracy organization, at a court hearing in June.

Andrei Pivovarov, chief executive of Open Russia, a pro-democracy organization, at a court hearing in June.

Photo: Tatiana Usmanova/Reuters

“Elections are coming soon and the authorities are ready to do anything to impede independent politicians,” its chief executive, Andrei Pivovarov, said at the time. He was removed from a plane bound to Warsaw from St. Petersburg in June and prosecutors subsequently charged him with managing an organization deemed undesirable, an offense that could carry up to six years in prison if he is convicted.

Lawmakers are also being squeezed. Pavel Grudinin, a high-profile Communist Party candidate who won 12% of the vote against Mr. Putin in the 2018 presidential election, was barred from running in September’s election after prosecutors said they found he owns shares in a foreign company. Dmitry Gudkov, who served in the lower house of parliament from 2011 to 2016 and had hoped to run for a seat again in September’s election, was prohibited from standing after participating in protests in support of Mr. Navalny earlier this year and because prosecutors filed criminal proceedings against him in relation to an unpaid debt allegedly owed by his aunt on a basement she rented over six years ago.

Russian opposition politician Dmitry Gudkov fled Russia in June.

Russian opposition politician Dmitry Gudkov fled Russia in June.

Photo: VALENTYN OGIRENKO/Reuters

Mr. Gudkov said authorities raided his home and those of family members and colleagues more than a dozen times before he fled the country for Bulgaria in June. Speaking by phone, he argued that the Kremlin is applying pressure not just to ensure the ruling United Russia party wins the September vote, but to deter possible protests against the results or the way the elections are handled.

The Kremlin has dismissed accusations that it is trying to purge political opponents ahead of the parliamentary vote. Presidential spokesman Dmitry Peskov told reporters last month that legal cases against Messrs. Gudkov and Pivovarov and other opposition figures “have nothing to do with politics.” He has also said the moves to identify opposition or media groups as foreign agents mirrors the U.S. Justice Department’s requirement that RT America, an affiliate of a Russian television company, register as a foreign agent in the U.S.

Political analysts, however, say the government’s maneuvers go far beyond its usual pre-election effort to clamp down on dissent.

“It’s beginning to seem that the elections are just an excuse to clean up the field of activists, as well as nongovernmental organizations and media that work independently of the state,” said Valentina Dekhtyarenko, an activist with the Zona Prava human rights group in Moscow.

In one example, Komanda 29, a well-known association of human rights lawyers, suspended its work earlier this month after authorities in Moscow ordered it to take down its website, citing a connection to Společnost Svobody Informace, a Czech-registered NGO, which is listed as an undesirable organization in Russia. The group complied.

“Sometimes you have to make very unpleasant and very difficult decisions in order to save the main thing: people,” said Ivan Pavlov, a lead lawyer with the group, in comments forwarded by his press secretary. He has been under criminal investigation since April, when prosecutors accused him of disclosing classified information relating to one of his clients, and is barred from using any communication devices.

Russia’s Prosecutor General’s Office didn’t respond to a request for comment on the measures it has taken against Mr. Pavlov and Komanda 29.

Yuliya Yarosh, editor in chief of Open Media, an online journalism outlet and sister organization of Open Russia, was on a separate foreign agents list for media this month along with two of her colleagues.

The publication is known for its high-profile investigations ranging from the assets of Mr. Putin and his inner circle to alleged corruption surrounding Mikhail Mishustin, the prime minister. Ms. Yarosh will now have to register as a legal entity in Russia and report her income and expenses from all sources to authorities every quarter, including how much she spends on food, clothes and gasoline. Everything she publishes or broadcasts, even on social media, must include the disclaimer “I am a foreign agent,” she said.

“Otherwise there will be fines, and after a several fines, a criminal case may be initiated,” Ms. Yarosh said.

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The register of foreign media agents, started in 2017, now includes 34 publications and journalists, according to Russia’s justice ministry. Seventeen names, including 13 individual journalists, have been added to the list since April 23.

Many of those targeted say the options for fighting back are limited, but they say they are determined to keep up the fight, despite the risks.

“Pulling ourselves back, engaging in self-censorship, thinking that we probably won’t write about the elections now because then we will be completely closed—this is nonsense,” Ms. Yarosh said, while acknowledging she would observe the rules of being labeled a foreign agent.

Exiled politicians and media groups working from abroad said they would continue trying to expose government malpractice and provide support to dissidents.

Mr. Gudkov and other activists said they would continue encouraging Russians to practice what they call smart voting, a strategy Mr. Navalny has championed where people are urged to vote for the strongest opponent to Mr. Putin regardless of which party they represent.

The difficulty is that fewer and fewer such candidates will make the ballot.

Related Video

Russian President Vladimir Putin told reporters following a June summit meeting with President Biden that jailed dissident Alexei Navalny had broken the law and would answer for his actions. Biden said he told Putin the consequences “will be devastating for Russia,” if Navalny dies in custody. Photo: Getty Images; Press Pool The Wall Street Journal Interactive Edition

—Valentina Ochirova contributed to this article.

Write to Ann M. Simmons at ann.simmons@wsj.com