The law restores Soviet-era powers to the Federal Security Service, the KGB's main successor agency, a move that rights advocates fear could be used to stifle protests and intimidate the Kremlin's political opponents. They also say the law's obscure wording leaves it too open to local interpretation.
The agency, known by its initials FSB, can now issue warnings or detain people suspected of preparing to commit crimes against Russia's security. Perpetrators face fines or up to 15 days of detention.
"It's an ugly law with obscure formulas," independent political analyst Yulia Latynina told The Associated Press.
"In case a drunken FSB officer is shooting at you - and there have been many such cases - you might end up getting jailed for 15 days for merely trying to escape."
The new law was described as part of an effort to combat extremism and thwart terrorist attacks. It was submitted to Russian lawmakers in April after twin subway bombings in Moscow killed 40 people and the Kremlin faced critical media coverage of its anti-terrorism efforts.
A senior lawmaker said the new powers will protect people from abuse by law enforcement officers - a significant problem in Russia.
"Officers of law enforcement agencies have long talked about the necessity of switching from investigating crimes to their prevention," Mikhail Margelov, the Kremlin-connected head of the foreign affairs committee in the upper house of Russian Parliament, said in a statement.
"The amendments do not turn the FSB into a new edition of the once-almighty KGB but protect Russian citizens from outrages by men in uniform."
Some of the law's most stringent sections - including ones that toughen control over the media for "extremist statements" or allow the FSB to publish warnings in the press - were removed or toned down following severe criticism from the opposition and even Kremlin loyalists.
Still, a lawmaker with the Communist Party, the largest opposition force in Russia's rubber-stamp Parliament, said the latest changes did not tone down the law's repressive character.
"Despite all the promises to correct the most odious articles, by the second reading nothing had been changed in the text," Viktor Ilykhin told the AP.
A Kremlin loyalist, meanwhile, praised the law for its "preventative measures."
"This is not a repressive law," Vladimir Zhirinovsky, leader of the nationalist Liberal Democratic party, told Gazeta.ru online daily. "We're only talking about preventive measures."
The opposition has accused the Kremlin of turning Russia into a Soviet-style police state, and many Russians say they have experienced or fear abuse at the hands of FSB officers. Government critics say corruption among the FSB and other agencies stifles business activity and stunts the economy.
Some rights activists say the law simply legalizes practices that FSB officers have been using for years.
"I don't think it adds anything to what the FSB has been doing without any laws," Lyudmila Alexeyeva, former Soviet dissident and head of the Moscow Helsinki Group, told the AP. "But it's very sad when a law approves the outrage of such a dangerous service as the FSB."
The legislation continues a trend under Vladimir Putin, who as president for eight years rolled back many of Russia's democratic reforms of the 1990s. The former KGB officer and FSB head allowed the security services to regain power and influence at the expense of Russia's democratic institutions.
Putin is now prime minister, and many see his intolerance of dissent as influencing Medvedev, his hand-picked successor.
The bill has raised doubts about Medvedev's commitment to promoting full-fledged democracy and freedom of expression.
Medvedev often has spoken of instituting judicial and police reforms, and has taken a less hard line on many issues than Putin.
Medvedev, who initiated the bill, responded angrily to criticism of it. He said earlier this month that "each country has the right to perfect its legislation."
