U.S. Secretary of State John Kerry on Sunday urged Russia to carry out a "thorough, transparent, real investigation" into the murder of Russian opposition leader Boris Nemtsov, a fierce critic of President Vladimir Putin.

Nemtsov was shot dead in Moscow on Friday, just two days before he was to have led a major opposition rally against the government.

Tens of thousands of people marched in central Moscow on Sunday to honor the memory of opposition leader Boris Nemtsov, who was gunned down near the Kremlin in the highest-profile assassination of Vladimir Putin's rule.

A sea of grim supporters, holding Russian flags and Nemtsov portraits, marched in drizzle from a packed Moscow square to the bridge over the Moskva where the 55-year-old was shot in the back shortly before midnight Friday.

A week since pro-Russian rebels forced Ukrainian soldiers from the key transport hub of Debaltseve, the sound of explosions still reverberates around the eastern town.

This, however, is not the boom of artillery fire and combat -- it is teams of Cossack deminers from the separatist side blowing up mines and unexploded ordnance littering this war-scarred railway junction.

Estonians voted Sunday in an election marked by jitters over a militarily resurgent Russia and a popular pro-Kremlin party, with the security conscious center-left coalition tipped for a return to power.

Moscow's annexation of Crimea last year and its meddling in eastern Ukraine have galvanized the European Union, including this eurozone member of 1.3 million people, a quarter of whom are ethnic Russian.

Ukraine's frontlines were relatively calm on Sunday ahead of high-level EU-mediated gas talks between Kiev and Moscow, as journalists mourned the killing by mortar fire of a Ukrainian photographer.

Kiev's security officials said there was no fire after midnight on Ukraine's positions and no Ukrainian soldiers have been killed over the past 24 hours. 

Western leaders and Russia's opposition on Saturday condemned the drive-by assassination of Kremlin critic Boris Nemtsov while President Vladimir Putin suggested the crime was staged to throw suspicion on the state.  

U.S. President Barack Obama decried the "brutal" and "vicious murder" of Nemtsov, which prompted the cancellation of a major opposition rally planned for Sunday, and urged Russia to conduct an impartial probe.

Exiled Russian former tycoon Mikhail Khodorkovsky warned on Friday that Vladimir Putin's policies in the Ukraine conflict will bring Russia to ruin, much as the Afghan war helped bankrupt the Soviet Union.

A year after annexing Crimea in Ukraine, Russia finds itself isolated and on a path to self-destruction, said Khodorkovsky, one of Putin's most outspoken critics, who spent a decade in prison on what he said were politically motivated charges.

The authorities in Russian-annexed Crimea said Friday they have nationalized film studios in the Black Sea resort of Yalta, famous for Soviet productions including Andrei Tarkovsky's classic "Solaris".

Crimea's State Council, the ruling regional body, passed a resolution saying the studios have been incorporated "into the property of the republic." 

Rebel fighters with Kalashnikovs watched over a dust-covered group of captured Ukrainian soldiers as they searched with their hands through the apocalyptic jumble of smashed concrete and twisted metal that is Donetsk airport.

The task of the 18 captives was a grim one.

Ukraine said on Friday three soldiers had been killed, the first fatalities in several days in the war-torn east, as a shaky truce with pro-Russian rebels appeared to gain some traction with an apparent weapons pull-back.

Isolated skirmishes highlighted the fragile situation as the U.N. discussed the conflict exactly one year after Russia and pro-Moscow forces began seizing ports and cities on the Black Sea peninsula of Crimea.