Opposition supporters will march through Moscow on Sunday in memory of Kremlin critic Boris Nemtsov, whose murder has increased concern about Russia's future among opponents of President Vladimir Putin.
Thousands of people laid flowers and lit candles on Saturday on a bridge near the Kremlin where the opposition politician and former deputy prime minister was shot dead late on Friday.
National investigators who answer to Putin say they are pursuing several lines of inquiry, including the possibility that Nemtsov, a Jew, was killed by radical Islamists or that the opposition killed him to blacken the president's name.
Putin's opponents say such suggestions show the cynicism of Russia's leaders as they whip up nationalism, hatred and anti-Western hysteria to rally support for his policies on Ukraine and deflect blame for an economic crisis.
"It is a blow to Russia. If political views are punished this way, then this country simply has no future," Sergei Mitrokhin, an opposition leader, said of Nemtsov's murder.
Putin has described the killing as a "provocation", and told Nemtsov's mother that the killers would be found and punished.