Moscow: Thousands of Russians prepared on Sunday to march through central Moscow in memory of opposition leader Boris Nemtsov who was gunned down near the Kremlin in the highest-profile assassination of Vladimir Putin's rule.
Friday's murder of the 55-year-old former first deputy prime minister under Boris Yeltsin came ahead of a major opposition rally he had planned to lead. It plunged his shocked supporters into mourning, and prompted international condemnation led by US President Barack Obama.
Putin swiftly decried the brazen shooting as a "provocation" and in a condolences telegram to Nemtsov's 87-year-old mother vowed to bring the perpetrators of "a vile and cynical murder" to justice.
Opposition activists scrapped plans to rally in southeast Moscow as authorities gave them rare permission to hold a 50,000-strong march to the bridge where Nemtsov was killed with four bullets to his back.
Shortly after the killing in sight of the Kremlin and the golden domes of Saint Basil's Cathedral, both prominent and ordinary Russians flocked to the scene, now laden with heaps of flowers, photos and notes.
Investigators said the murder of the politician known for his criticism of Moscow's involvement in Ukraine had been meticulously planned.
The Investigative Committee, which reports directly to Putin, said Nemtsov "might have been sacrificed" to sow instability, and they were also checking any links to the Ukraine conflict.
Opposition and liberal-minded Russians said that even if authorities were not involved in the murder, the Kremlin bore direct responsibility for whipping up anti-Western hysteria and hatred against dissenters which made the assassination possible.
Kremlin critics said the murder of the top opposition leader who had held senior government posts marked a watershed moment in politics.
"We have entered a new epoch -- the epoch of the physical liquidation of political opponents of the regime," prominent commentator Yulia Latynina said on radio.
"The message is absolutely clear: anyone who attends an opposition march can be killed. This is an act of political terror."
The Russian president's spokesman swiftly sought to allay fears.
"It would be too emotional and wrong to conclude that a string of such murders has begun," Dmitry Peskov said in televised remarks.
Last weekend tens of thousands of Kremlin supporters including radical bikers and former soldiers were mobilised to demonstrate near the Kremlin walls, vowing to protect Putin from "Russia's enemies."