The legislation, signed into law earlier this month by Russian President Vladimir Putin, had already drawn scorn from critics in and outside of Russia.
Known as the "Yarovaya Law," the measure includes new police and counterterrorism measures that directly echo the sweeping powers wielded by the KGB to stifle dissent and repress opposition activists throughout the Soviet era.
But one largely overlooked aspect of the law is garnering new scrutiny and worry: tight restrictions on the activities of religious groups, particularly smaller denominations.
The new restrictions "will make it easier for Russian authorities to repress religious communities, stifle peaceful dissent, and detain and imprison people," said Thomas J. Reese, who heads the U.S. Commission on International Religious Freedom, a federal government agency that monitors religious expression around the world.
"Neither these measures nor the currently existing antiextremism law meet international human rights and religious freedom standards," he said in statement released last week.
Since the breakup of the communist Soviet Union 25 years ago, Russia's main religious faiths have flourished, with the largest denomination, the Russian Orthodox Church, now awash in money and believers. A law passed in 1997 officially named Orthodox Christianity, along with Islam, Judaism, and Buddhism, as the country's four "traditional" faiths.
After Orthodoxy, Muslims make up the second-largest religious group in Russia, and state funds have been used to help build mosques from Chechnya to Tatarstan.
Other major Christian denominations like the Roman Catholic Church have also been allowed to operate openly and largely without restrictions, though the Vatican and Russian Orthodox leaders have clashed in the past over ownership of church property dating back to the Bolshevik Revolution.
But denominations with a smaller presence in Russia -- Protestants or Jehovah's Witnesses, for example -- have long been viewed with hostility from state officials and religious authorities, and many have long complained the 1997 law set up registration and administrative procedures that were onerous and expensive to comply with.
The law signed by Putin, which takes effect on July 20, is ostensibly aimed at tightening measures in the fight against terrorism.
Among its most controversial provisions, the law increases security agencies' access to private communications, requiring telecom companies to store all telephone conversations, text messages, videos, and picture messages for six months and make this data available to authorities.
But the law also puts more restrictions on religious groups' activities in the name of fighting "extremism," a term that rights activists have long complained is so broad and ill-defined that any manner of dissent or unsanctioned protest could be criminalized.
For religious groups, the new law requires people to get official permits through a registered religious group and bars things like prayer meetings from taking place anywhere except for officially recognized religious buildings. That would potentially forbid house churches.
Members of a religious group would also potentially be barred from e-mailing invitations to people interested in services, according to Christianity Today, a web-based news service focused on religious issues.
Violators could be fined, or potentially expelled from Russia.
The Church of Jesus Christ of Latter-Day Saints, the Utah-headquartered denomination known widely as the Mormons, issued a statement on July 8 suggesting concern with the law, saying it "will have an impact on missionary work." Mission work, which involves members, typically young people, spreading information about the church, is a central precept for the denomination.
"The church will honor, sustain, and obey the law," said the organization, which has around 23,000 members in Russia. "The church will further study and analyze the law and its impact as it goes into effect."
Sergei Ryakhovsky, a Pentecostal church leader and co-head of an organization of Protestant churches in Russia, said in an open letter co-signed by him that the law contradicted the Russian Constitution.
"The obligation on every believer to have a special permit to spread his or her beliefs, as well as hand out religious literature and material outside of places of worship and used structures, is not only absurd and offensive, but also creates the basis for mass persecution of believers for violating these provisions," said the letter, which was posted on the Russian-language religious website Portal-Credo.
"This law brings us back to a shameful past," it said.