Under the new law, anyone possessing information deemed secret — whether a politician, a journalist, an environmentalist or a union leader — could potentially be jailed for up to 20 years for espionage.
Human rights advocates say the law is part of a widening crackdown on the opposition after President Vladimir Putin began his third presidential term in May.
"It's very broad and it's very dangerous," Rachel Denber, deputy director of Human Rights Watch's Europe and Central Asia division, told The Associated Press.
She said it's not clear yet how vigorously Russian authorities will enforce the bill, but says it recreates a "sense of paranoia and suspicion and uneasiness about foreigners."
While the previous law described high treason as espionage or other assistance to a foreign state that damages Russia's external security, the new legislation expands the definition by dropping the word "external." Activities that fall under it include providing help or advice to a foreign state or giving information to an international or foreign organization.
The law, which was drafted by the Federal Security Service, the main KGB successor agency known under its Russian acronym of FSB, also introduced a punishment of up to eight years for simply getting hold of state secrets illegally even if they aren't passed to foreign hands.
The FSB explained in a statement run by the ITAR-Tass news agency that the new clause better protects confidential information.
It said the previous law, which dated back to the 1960s, failed to provide an efficient deterrence against foreign spies.
"Tactics and methods of foreign special services have changed, becoming more subtle and disguised as legitimate actions," the spy agency said. "Claims about a possible twist of spy mania in connection with the law's passage are ungrounded and based exclusively on emotions."
Tamara Morshchakova, a former Constitutional Court judge, told a presidential rights council meeting Monday that the new law is so broad the FSB no longer needs to provide proof that a suspect inflicted actual damage to the nation's security.
"Their goal was simple: We have few traitors, it's difficult to prove their guilt, so it's necessary to expand it," Morshchakova said. "Now they don't have to prove it any more. An opinion of law enforcement agencies would suffice."
Putin, who chaired the meeting, promised to take another look at the treason bill to prevent it from being excessively broad, but it became law despite his pledge.
Putin's spokesman Dmitry Peskov said Wednesday, in comments carried by Russian news agencies, that Putin's statement indicated his readiness to review the treason law if its implementation reveals "some problems or aspects restricting rights and freedoms."
The revised treason bill first came up in 2008, under then-President Dmitry Medvedev, who quickly shelved the bill after an outburst of public criticism.
Putin, a KGB veteran, has clamped down on his critics following a series of huge street protests in Moscow against his re-election, which he said were staged by Washington in order to weaken Russia. The Kremlin-controlled parliament quickly stamped a series of repressive bills and opposition activists have faced numerous searches and arrests.
"There is an effort to recreate an old sense of fear," Denber said, adding that the new legislation was apparently aimed at discouraging Russians from joining protests. "One of the aims is surely to never have that happen again and to demonize any ... people or organization that might be associated with that."
One of the laws passed this summer required non-governmental organizations in Russia that receive foreign funding and engage in political activity to register as "foreign agents," which aims to destroy their credibility among Russians. One such group is Golos, Russia's only independent vote monitor, which collected evidence of massive violations in recent elections.
And in October, Moscow ended the U.S. Agency for International Development's two decades of work in Russia, saying the agency was using its money to influence Russian elections — a claim the U.S. denied.
Denber said her group already felt a new chill on a recent visit to one of Russia's Siberian provinces while doing a research on health care. Local officials demanded to know who invited them, who paid for the trip and the names of the group's local contacts.
"It was very hard, it was an echo of a different time," she said.