The 55-year-old former deputy prime minister, a longtime Putin critic and renowned anti-corruption crusader, was shot dead shortly before midnight Friday while walking across a bridge just a short distance from the Kremlin.
Shocked opposition figures in Russia and Western leaders called for a full and transparent probe into the murder of Nemtsov, who served as Boris Yeltsin's first deputy prime minister in the 1990s.
Tomorrow, Nemtsov's body will lie in state at the Andrei Sakharov rights centre in Moscow, followed by his burial at the city's Troekurovskoye cemetery.
Putin himself vowed to spare no effort to bring the killers to justice, ordering law enforcement chiefs to personally take on the case.
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Foreign Minister Sergei Lavrov pledged Monday that the "heinous crime" would be "fully investigated."
A reward of three million rubles (USD 48,000) is on offer for information on Nemtsov's death, a substantial amount in Moscow, where the average monthly salary is 60,000 rubles (USD 960).
Various motives for the murder have been floated, including Nemtsov's opposition to Russia's role in the Ukraine conflict, as well as his condemnation of January's killings at the Charlie Hebdo satirical weekly in Paris by Islamist gunmen.