For Mr. Putin, the jousting over Ukraine is the latest of several foreign policy moves that have served to re-establish Russia as a counterweight to Western dominance of world affairs. This year, he defied Washington by granting temporary asylum to Edward J. Snowden, the former National Security Agency contractor, and deflected an American military strike on his longtime ally, President Bashar al-Assad of Syria, with a proposal to eliminate Syria’s chemical weapons.
There was no immediate quid pro quo for Russia — at least not in plain sight — as Mr. Putin announced the deal at the Kremlin with Ukraine’s embattled president, Viktor F. Yanukovich. Protesters in Kiev have been deeply worried that Mr. Yanukovich would cut a secret deal to join a customs union that Russia has established with Belarus and Kazakhstan. The union is essentially a free-trade zone across a large section of the former Soviet Union, allowing goods to cross travel through borders without clearing customs.
Over the weeks of protests, however, it became clear that the customs union was a nonstarter for Ukraine, and Mr. Putin said the subject did not come up in their discussions on Tuesday.
In Independence Square, where the large crowd was bolstered by people coming out of work, the initial reaction appeared to be a mix of fury and dismay, with people chanting, “Out with the crook!” But there was no call for drastic new steps.
Leaders of the three opposition parties who are coordinating the protest said the demonstrations would continue, and they voiced suspicions about what Mr. Yanukovich had offered in exchange for a Russian bailout.
“Free cheese is only found in a mousetrap,” Arseniy P. Yatsenyuk, the leader of the Fatherland coalition in Parliament, said in a speech. He asked for the patience of protesters.
The implications for the protest movement were not immediately clear, but Mr. Putin’s announcement, at a Kremlin meeting with Mr. Yanukovich, substantially alters the political landscape. It throws Mr. Yanukovich an economic and political lifeline that will spare him for now from negotiations with the International Monetary Fund, which was demanding significant changes to the government, judiciary and the economy in exchange for aid.
While Mr. Putin portrayed Russia’s assistance as a gallant move, requiring Ukraine neither to commit to the customs union nor to put in place any of the austerity measures demanded by the I.M.F., the rescue plan carries serious long-term economic and political risks. Experts say that unless Ukraine carries out overhauls, including increases in household utility rates, limits on government spending and pension increases, and improvements in the business climate, the country’s economic problems will continue, raising the likelihood that the aid will be wasted.
In addition, the political uncertainty raises the possibility of changes at the highest levels of government, perhaps even sooner than the presidential elections in February 2015.
Then there is the immediate investment risk. Russia’s rules for its national welfare fund require investments in countries with long-term bond ratings of AA or better; both Fitch and Standard & Poor’s rate Ukraine at B-.
Mr. Putin, sitting beside Mr. Yanukovich on Tuesday, said Russia was happy to help. “With the goal of supporting the budget of Ukraine, the government of the Russian Federation made the decision to issue in bonds from the Ukrainian government part of its own reserves from the national welfare fund in the amount of $15 billion,” he said.
Nodding to the demands in the West, he added: “I want to bring your attention to the fact that it is not connected with any conditions, not connected with the increase, decrease or freezing of any social standards, pensions, subsidies or salaries.
“And I want to calm everyone down. Today we did not discuss the question of Ukraine’s accession to the customs union.”