MOSCOW—New restrictions signed into law over the weekend by President Vladimir Putin allow Russian authorities the power to shut down foreign-backed groups deemed "undesirable," in a step activists say is likely to further pressure the country's beleaguered civil society.

The new law allows any foreign or international NGO to be shut down and introduces cash fines, restrictions on movement or jail time of up to six years for those who violate it. It builds on a law passed in 2012 that branded groups as "foreign agents" for their supposed political activities as well as funding they received abroad. About 60 organizations have been officially listed in that category.

The term "undesirable" hasn't been widely used in Russia before, and in the legislation it is only vaguely defined, leaving room for broad interpretation. Critics of the law said that lack of a clear definition will allow for abuse. The new law is the latest step in what rights groups say is the Kremlin's policy of clamping down on critical voices, some of whom have been accused of being pawns in a Western plot to repeat a Ukraine-style revolution in Russia.

Pavel Chikov, head of a group that provides legal services to nongovernmental organizations, said Monday the new law increases the scope and severity of the foreign agent law, which has sent authorities to the doorsteps of numerous rights organizations, including Amnesty International and groups probing the alleged deaths of Russian troops in Ukraine.

"The law allows the forbidding of any social and political activity once it is declared an 'undesired' organization, and those who continue to work can be sent to prison," said Mr. Chikov, whose own group, Agora, has been declared a foreign agent.

"Most of all, this is about rights advocacy organizations, ecologists and all other groups that champion liberal values, because the Kremlin considers them a risk to the current political regime," he added.

Lawmakers who introduced the bill, which was passed quickly in three readings with little debate, said when it was first brought before parliament last year that it was meant to prevent outside forces from instigating a violent overthrow of the government.

"It is unacceptable that one of our foreign organizations would have the ability to distribute weapons as was the case in Ukraine," said lawmaker Anton Ishchenko, speaking to a Russian newspaper at the time.

Mr. Chikov said lawmakers were already looking into organizations including Human Rights Watch, Transparency International and Memorial, Russia's oldest human rights group.

The U.S. quickly criticized the law, which allows prosecutors to declare an organization "undesirable" without court proceedings.

"We are concerned this new power will further restrict the work of civil society in Russia and is a further example of the Russian government's growing crackdown on independent voices and intentional steps to isolate the Russian people from the world," State Department spokeswoman Marie Harf said.

Tanya Lokshina, senior researcher for Human Rights Watch in Moscow, said the law was aimed not at closing organizations down, so much as raising the stakes for Russians who want to work in groups that could be critical of the government.

Mr. Putin supported the foreign agent law after vote-monitoring group Golos cataloged numerous alleged voting violations during the 2010 parliamentary election. Anger at the irregularities sparked some of the biggest protests in Russia since Mr. Putin took over the presidency in 2000.

Pro-Kremlin media has run purported exposés on pro-democracy groups receiving money to inspire a revolution in Russia, like those that toppled Middle East and North African regimes during the Arab Spring.

Write to Thomas Grove at thomas.grove@wsj.com

Subscribe to WSJ: http://online.wsj.com?mod=djnwires