According to Russian-language news website Meduza and Interfax, Nemtsov was shot four times in the back as he walked near the Kremlin with a woman. His assailants reportedly fled the scene in the white car.
— Yury Barmin (@yurybarmin) February 27, 2015
Mr Nemtsov, 55, served as first deputy prime minister under the late President Boris Yeltsin in the 1990s.
He had earned a reputation as an economic reformer while governor of one of Russia's biggest cities, Nizhny Novgorod.
Falling out of favour with Yeltsin's successor, Vladimir Putin, Mr Nemtsov became an outspoken opposition politician.
In 2013, CNN reports, Nemtsov called the 2014 Winter Olympics in Sochi one of the most "outrageous swindles" in modern Russian history and claimed that some $30 billion of the games' total cost had been embezzled.
[Image via AP]