Boris Nemtsov, a towering figure in Russian post-Soviet politics and a biting critic of President Vladimir Putin, was gunned down steps from the Kremlin late Friday, authorities said. The drive-by shooting had the potential to open a violent new chapter in Russian political life.

Nemtsov, a physicist-turned-politician who was seen in the 1990s as a possible heir to President Boris Yeltsin, was one of the loudest voices condemning Russia’s sharp turn toward confrontation with the West in the past year. The killing sent immediate shock waves through Russia, where he became the highest-profile opposition leader to be slain in a nation where such figures are sometimes imprisoned or pushed to emigrate.

There was no immediate information on who killed the 55-year-old politician as he walked across a central Moscow bridge shortly before midnight on an unusually warm winter’s night. Putin said it bore the marks of a contract killing intended to embarrass the Kremlin, a spokesman said. Opposition leaders said they were sure that it was an attempt to intimidate them.

The killing was a dramatic and bloody turn for Russia’s oppressed opposition movement, which has struggled to find its footing during a wave of nationalistic fervor unleashed by the annexation of Ukraine’s semi­autonomous Crimean region last year. Many leaders have been marginalized with prison terms or other forms of harassment, and public rhetoric has grown extremely aggressive toward those who deviate from the majority line.

The shooting came a day before a rally at which opposition leaders had been hoping to breathe fresh life into their cause. Nemtsov was one of the main organizers.

Politically motivated slayings are not unknown in Russia, but not once in the 24 years since the breakup of the Soviet Union has such a high-profile figure been the victim. Shaken opposition leaders said in the hours after Nemtsov’s slaying that they were newly fearful, but they vowed to carry on. It was not immediately clear whether they would hold the rally on Sunday.

Just hours before his death, Nemtsov told Ekho Moskvy radio that Putin had pushed Russia into an economic crisis through his “mad, aggressive and deadly policy of war against Ukraine.”

The killing took place as Nemtsov walked in the heart of Moscow across the Bolshoi Moskvoretsky Bridge, less than 100 yards from the walls of the Kremlin and within sight of Red Square, one of the most secure areas in all of Russia. Police and secret services­ have a heavy presence in the region. There was no word on whether Putin was in the Kremlin at the time; he typically sleeps at a presidential residence on the outskirts of Moscow.

“Nemtsov’s murder is a terrible tragedy for Russia,” said former finance minister Alexei Kudrin, one of the few Putin allies who is also publicly critical of him.

Nemtsov was a political star in the early post-Soviet days, when most Russians still dreamed of democracy — a young, energetic and smart politician who charmed voters and won high approval ratings as a regional governor and then as Russia’s deputy prime minister. For a time, he was seen as a likely heir to Yeltsin, who served from 1991 to 1999 as the first president of the Russian Federation.

Instead, Putin assumed the presidency in December 1999 and set about relentlessly marginalizing his opponents. He has held power in one capacity or another since then.

Nemtsov received a doctorate in physics in 1990 and then served as a lawmaker for three years. Yeltsin appointed him governor of Nizhny Novgorod province in 1991.

He was a key architect of Russia’s no-holds-barred moves toward capitalism, instituting free-market economic policies and simplifying the nightmarish and corrupt processes of registering new businesses.

Nemtsov was so popular that the Yeltsin camp of reformers briefly considered running him for president in 1996, but nothing came of the effort. Nemtsov reluctantly accepted the office of first deputy prime minister after Yeltsin was reelected in 1997.

As deputy prime minister, Nemtsov spearheaded a program of economic “shock therapy” designed to haul Russia out of its post-Soviet doldrums. He sought to make bidding on government contracts more competitive and transparent. His efforts earned him the ire of Russia’s notorious oligarchs, the powerful businessmen who were accused of looting the nation’s assets after the fall of the Communist government.

More recently, Nemtsov had been working on a report that he said would prove that Russian soldiers were fighting alongside pro-Russian rebels in eastern Ukraine, where a bloody 11-month-conflict has claimed nearly 5,800 lives. The Kremlin has hotly denied any direct involvement, which Russian opinion polls suggest would be deeply unpopular.

He had angered the government two years ago when he charged that billions of dollars had been stolen from funds designated for the 2014 Winter Olympics in Sochi, his home town. He blamed “Putin’s friends” for the alleged embezzlement, which he described as “a real threat to Russia’s national security.”

Flashing police lights illuminated the night Saturday on the Bolshoi Moskvoretsky Bridge, which runs from the base of St. Basil’s Cathedral to an island in the Moscow River where major opposition rallies were held in 2011 and 2012. Nemtsov was shot four times, Interior Ministry spokeswoman Yelena Alexeeva told journalists at the scene. Authorities were questioning witnesses, including a Ukrainian woman with whom Nemtsov was walking across the bridge when he was shot, Alexeeva said.

Images broadcast after his slaying showed his body lying face-up on the sidewalk of the bridge as emergency personnel appeared to attend to him.

Putin quickly condemned the killing and said he was directing Russia’s top security officials to take personal charge of the investigation, a measure of the shock waves that it sent through the political establishment.

“Putin noted that this cruel murder has every sign of being a contract killing, which has a solely provocative nature,” Kremlin spokesman Dmitry Peskov told Russian news agencies.

Russian opposition leaders said Saturday that they were fearful that Nemtsov’s slaying was only the first in a new era of political repression.

“This murder, politically, it hits the spot, because if the message is to send a scare throughout the opposition movement, this is one thing to do,” said Vladimir Milov, an opposition leader who alongside Nemtsov had been planning the Sunday rally.

“Scared or not scared, we will carry on,” he said.

Milov said he did not think it would be the last killing of an opposition leader in Russia.

“This is a new level, but as sad as it may sound, we have to expect a continuation. Because this is just how Russia operates now,” he said.

In 2012, Putin warned publicly that his opponents were prepared to murder one of their own so they could blame him for the death.

“They are looking for a so-called sacrificial victim among some prominent figures,” Putin, a former KGB agent, told a gathering of the All-Russia Popular Front, a group organized to support him, ahead of Russia’s 2012 presidential election. “They will knock him off, I beg your pardon, and then blame the authorities for that.”

In comments that took on a new significance Saturday, Nemtsov said earlier this month that he was worried that Putin would have him killed.

“A bit” worried, Nemtsov told the Sobesednik magazine. “Not as much as my mother, but still.”

“If I were very fearful,” he said, “I probably wouldn’t head an opposition party. I probably wouldn’t be involved in what I do.”

Branigin reported from Washington. Kathy Lally in Washington and Natasha Abbakumova in Moscow contributed to this report.

CORRECTION: According to investigators, Boris Nemtsov was killed shortly before midnight Friday. The timing of his death has been updated in this article.

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