A group of European vote monitors said Saturday they were denied a meeting with Russian strongman Vladimir Putin ahead of next month's presidential elections because of his busy schedule.

The Russian premier is the overwhelming favorite to beat four weak rivals in a March 4 ballot that should hand him a third term as Kremlin chief.

A Russian space center engineer received a 13-year jail sentence Friday on charges of passing secret strategic missile data to the U.S. Central Intelligence Service (CIA) in return for cash.

Russia's FSB security service said Plesetsk launch site engineer Vladimir Nesterets admitted receiving cash payments for information about "tests on Russia's latest strategic missile systems."

Russian leaders under fire for a U.N. veto Wednesday rejected outside interference in the Syrian conflict, with Prime Minister Vladimir Putin warning against behaving "like a bull in a china shop."

"Of course we condemn violence from whichever side it comes, but we must not behave like a bull in a china shop. We need to allow people to decide their own fate independently," Putin said in televised remarks.

Prime Minister Vladimir Putin, facing an outburst of protest against his rule, called Monday for an update of Russia's political system in response to what he said was a maturing civil society.

"We need to create a political system where people can and must speak the truth," Putin said in a wordy article which also quoted Soviet dissident writer Alexander Solzhenitsyn.

The movement against Vladimir Putin was Sunday preparing new rallies to loosen the Russian strongman's 12-year grip on power after keeping up their challenge with a third mass demonstration in Moscow.

Tens of thousands marched through Moscow Saturday despite bitterly cold weather to protest against Putin's domination, with organizers disputing police claims that a pro-Putin rally elsewhere in the capital was even larger.

Russia's state-controlled Gazprom natural gas giant acknowledged for the first time Saturday that it had briefly reduced gas supplies to Europe amid a spell of extreme cold.

Gazprom deputy chief Andrey Kruglov reported to Prime Minister Vladimir Putin that the cuts lasted for several days and reached up to 10 percent, but supplies are currently back to normal. Officials in Austria and France, however, have reported cuts of as much as 30 percent, and Italy said supplies were down by 24 percent Thursday.

Tens of thousands took to the streets of Moscow Saturday for rival rallies opposing and supporting Russian strongman Vladimir Putin's political domination one month ahead of presidential polls.

Police said between 87,000 and 90,000 turned up for the pro-Putin rally in the west of the capital and put the number of protesters at the anti-Putin march to a square overlooking the Kremlin at around 23,000.

The leader of Russia's Islamist rebels ordered a halt to attacks on civilians, saying mass opposition protests showed the public no longer supported Vladimir Putin, according to a video posted Friday.

Doku Umarov, whose Caucasus Emirate group claimed atrocities including a deadly Moscow airport bombing in 2011 and a metro attack in 2010, said from now only security objects and officials would be targeted.

Russian strongman Vladimir Putin refused to rule out on Wednesday a run-off in March presidential polls where he will seek a third Kremlin term but warned a second round risked causing instability.

"I understand that a second round run-off is possible, according to the current legislation," Russian news agencies quoted Putin as saying at a meeting with Russian election observers.

Supporters of Vladimir Putin Tuesday called on people to attend a "counter-revolutionary" rally in Moscow on the same day as Russia's opposition holds its third mass protest in the Russian capital.

Organizers called on "anti-Orangist forces" to mount a show of strength against Saturday's opposition rally, in a reference to Ukraine's 2004 Orange Revolution that ousted its old order from power and infuriated the Kremlin.