Russian Prime Minister Vladimir Putin struck out hard at the United States on Thursday, signaling that the warming relations between the two countries could be sacrificed as he tries to bolster his authority in the face of street protests.

Putin lacerated Secretary of State Hillary Rodham Clinton for questioning the validity of last Sunday’s parliamentary elections and suggested that she had galvanized thousands of protesters by declaring the vote “neither free nor fair.”

“She set the tone for some of our public figures inside the country, sent a signal to them. They heard this signal and launched active work with the U.S. State Department’s support,” he said.

Clinton, in Brussels attending a meeting of NATO foreign ministers, tried to strike a conciliatory note but did not back away from her critiques of election irregularities.

“We value our relationship with Russia,” Clinton said. “At the same time . . . we expressed concerns that we thought were well-founded about the conduct of the elections.”

Although Russian officials regularly make critical remarks about the United States, and vice versa, Putin’s words were an escalation, harking back to the first years of his decade in power. But Russia watchers consider them aimed at domestic politics. Putin, they say, always needs an enemy as a foil.

Still, a serious rupture between the United States and Russia could have wide-reaching consequences. The reset in relations has brought the United States a number of rewards, including cooperation on fighting terrorism, permission to use Russian territory to supply troops in Afghanistan, agreement on the New START nuclear arms pact and cooperation on dealing with Iran.

And, as Putin reminded sternly Thursday, Russia is a nuclear nation with an attitude.

“We are a major nuclear power and remain such. And this causes some concerns for our partners,” he said, suggesting that the United States was throwing its weight around “so that we don’t forget who is the boss on our planet.”

From all appearances, Putin has been in utter control since assuming the presidency from Boris Yeltsin in 2000, even as he turned the office over to Dmitry Medvedev in 2008 because of term limits and became the power behind the throne as prime minister.

But cracks in the facade began to appear Sept. 24, when Medvedev informed Russians that he would return the crown to Putin, who would run for the presidency in March. Ordinary Russians who had given silent consent to Putin’s authority by ignoring it were insulted that their assent had been so carelessly assumed.

The week before the parliamentary elections, Putin made dark references to Golos, an independent election monitor, accusing it of acting for U.S. interests. He attacked Golos, critics said, to discredit eventual reports of fraudulent elections and prevent questions about the legitimacy of his party, United Russia, which controls not only the parliament but also local governments throughout the country.

Observers reported widespread irregularities after the vote, in which United Russia won just less than 50 percent. Monday night, about 5,000 protesters demonstrated against Putin and United Russia, crying fraud. The heavy police presence that accompanies any public gathering moved in, and about 300 were arrested.

On Tuesday, thousands demonstrating for Putin’s United Russia were left alone, while 600 opposition protesters were detained. More than 20,000 people have signed a Facebook page saying they plan to demonstrate Saturday against the government.

With his legitimacy under attack, Putin’s tone was harsh as he spoke Thursday to the Popular Front, a movement he created in May as a campaign vehicle.

“We need to safeguard ourselves from this interference in our internal affairs and defend our sovereignty,” Putin said. “It is necessary to think about improving the law and toughening responsibility for those who take orders from foreign states to influence internal political ­processes.”

Missile defense targeted

The government’s rhetoric gathered steam as the elections approached and intensified dramatically when Medvedev made the first serious threats about a Europe-based missile defense system.

In Brussels at the meeting of NATO foreign ministers Thursday, Russian officials said the alliance was trying to bully their country into participating in the system, which Moscow fears could be used against Russia. The United States says it is intended to curtail threats from elsewhere.

Ominous signs were mounting when an Interior Ministry official suggested that a cherished U.S. interest — an unfettered Internet — was ripe for control in Russia, saying that users should be identified by their names and faces. His boss quickly denied any such plan was under consideration.

The Vkontakte social network said that federal security services had asked it to block the activity of opposition groups. It refused.

Over the weekend, the Web sites of independent organizations reporting on the election were hacked, Golos among them. On Thursday, Golos Deputy Director Grigory Melkonyants said his Facebook page was under attack.

Late Wednesday, Yevgenia Chirikova, the widely admired leader of an environmental group that has been fighting the government, was detained at an airport, where she underwent a body search. She had been in Europe discussing the elections’ validity with officials.

“They searched and released,” she tweeted. “They did not find millions from the state department. They seem to be upset.”

At a news conference Thursday, the Interior Ministry insisted that Sergei Magnitsky, a whistleblowing lawyer working for an American firm, had died of heart failure while in pretrial detention in 2009 despite detailed reports finding that he was tortured and killed. The officials said they might try William Browder, the investment manager who employed Magnitsky, in absentia on tax charges.

The U.S. ambassador to Russia, John Beyrle, has been expected to leave this month, although his successor, Michael McFaul, the adviser to President Obama who was the architect of the reset, has not been confirmed.

Resetting the reset button

The Obama administration has shown signs of a less tolerant approach to Russia, suggesting it had met its reset objectives and was preparing for a testier relationship.

At the end of October, Clinton’s chief technology aide visited Russia to promote the benefits of a free Internet. Her assistant secretary for democracy and human rights met beleaguered activists, asking what kind of support the United States could provide.

Those who watch Russia expect that whatever happens, the consequences will be worse for Russians than for Americans.

Leon Aron, director of Russian studies at the American Enterprise Institute, said Putin faces some choices he might have wished to avoid.

“What I think dismays him is that until now it was fairly soft authoritarianism,” Aron said by telephone from Washington. “The dilemma he faces is embracing the more traditional authoritarianism or risk looking weak. In the last 12 years, the regime has not had to face this dilemma yet.”

Staff writer Karen DeYoung in Brussels contributed to this report.