Russian political opposition figure Boris Nemtsov was shot and killed in Moscow early Saturday, authorities said.
Investigative Committee spokesman Vladimir Markin said Nemtsov, an opposition politician and former deputy prime minister, was gunned down on a bridge near the Kremlin by someone inside a car who fired seven shots. Police said he was struck four times.
Interior Ministry spokeswoman Yelena Alexeyeva said Nemtsov was on a stroll with a female acquaintance, a Ukrainian citizen, from St. Basil's Cathedral when he was killed. The woman was not hurt.
Nemtsov's death comes just a day before a planned protest against President Vladimir Putin's rule. The Kremlin said that Putin will personally oversee the investigation.
Nemtsov was a sharp critic of Putin, assailing the government's inefficiency, rampant corruption and the Kremlin's policy on Ukraine, which has strained Russia-West ties to a degree unseen since Cold War times.
Nemtsov was a veteran member of the liberal opposition, and was briefly named as Boris Yelstin's possible successor in the late 1990s. He co-led mass protests in Moscow in 2011 through 2013.
The White House National Security Council called on the Russian government to conduct a prompt and transparent investigation into the killing, and to "ensure those responsible are brought to justice."
U.S. Rep. Ed Royce a Repulican from California and chairman of the House Foreign Affairs Committee, said, "Lawlessness now pervades Putin's Russia."
"Regardless of who killed Boris Nemtsov, this shocking murder is the latest assault on those who dare to oppose the Putin regime," Royce said in a statement. "The United States must strongly support all those brave individuals who refuse to be silenced by fear and who knowingly risk their lives to bring freedom to Russia."
The chess champion and fellow opposition figure Garry Kasparov said he was "devastated" by the killing. "Boris's quality no longer fit Putin's Russia. He always believed Russia could change from inside without violence; after 2012 I disagreed," Kasparov said on Twitter.
"When we argued, Boris would tell me I was too hasty, that in Russia you had to live a long time to see change," Kasparov said. "Now he'll never see it. RIP."
Additional reporting by The Associated Press