The four candidates that are challenging Vladimir Putin in the March 4 presidential elections include the Communist Party boss, a billionaire, a bearded populist and an ultra-nationalist.

As Russia edges toward Sunday's polls that will determine its leader for the next six years, the swelling protest movement has not rallied around a single candidate to oppose Putin's re-installment in the Kremlin.

Prime Minister Vladimir Putin said Tuesday he was unafraid for his life and had grown used to plots to kill him, after state television reported a new conspiracy days before Russia's presidential poll.

"You cannot live with constant fear -- let them fear us. I have been living with this since 1999," when Putin first became prime minister before heading the Kremlin in 2000-2008, Russian news agencies quoted him as saying.

Russia on Monday slammed as "one-sided" last week's Friends of Syria meeting in Tunis that condemned Damascus for its crackdown and vowed further sanctions against the Syrian regime.

"The meeting that was held in Tunis was clearly one-sided... It is clear to us that this meeting did not help create conditions that would stimulate all sides to seek a political solution," Foreign Minister Sergei Lavrov told reporters.

Beijing on Monday hit back at Hillary Clinton over her criticism of China and Russia's stance on Syria, calling the U.S. Secretary of State's comments unacceptable.

Clinton said Friday that the international community must work to change the positions of Moscow and Beijing, which have faced intense criticism for vetoing two U.N. resolutions condemning the Syrian regime.

For the past two decades, the U.S. space agency has been practically obsessed with Mars. It has hardly missed an opportunity about every two years to fling robotic spacecraft at the red planet.

This summer, the most high-tech rover ever, Curiosity, will land near the Martian equator in search of the chemical building blocks of life. The more scientists study Mars, the closer they get to answering whether microbial life once existed there, a clue to the ultimate question: Are we alone?

Russia's new Internet-savvy opposition is going online to protest and monitor the presidential elections on March 4, bringing its iPhones and Twitter into the fray against Vladimir Putin.

As jokes and spoof videos about Putin, expected to win back the presidency in Sunday's polls, spread like wildfire on social networking sites and YouTube, opposition activists are using the Internet to promote their cause.

Russia's secret service has arrested two men in connection with a plot to assassinate Prime Minister Vladimir Putin after the March 4 presidential elections, Channel One state television said Monday.

The station showed two men who said they were acting on the orders of Chechen warlord Doku Umarov. They said they prepared the attack in the Ukrainian port city of Odessa and were planning to carry it out in Moscow.

Thousands of Russians linked hands Sunday around Moscow in a symbolic protest against Prime Minister Vladimir Putin's expected return to the Kremlin for a third term in elections next weekend.

A din of endless honking descended on Moscow's 16-kilometer (10-mile) Garden Ring Road as drivers expressed support to large crowds of smiling and waving people who gathered in freezing weather under a heavy gray sky.

A strong earthquake measuring 6.8 shook southwestern Siberia in Russia on Sunday but caused no casualties or serious damage, the Russian emergencies ministry said.

The quake's epicenter was recorded at 1:20 pm local time (0620 GMT) in the republic of Tuva about 107 kilometers (66 miles) from the town of Kyzyl, which saw tremors of a similar magnitude on December 27.

Russian protest leader Alexei Navalny led thousands through the streets of Vladimir Putin's native city Saturday in protest against his likely return to the Kremlin in March 4 polls.

The demonstration was called a day before thousands more hoped to link hands around Moscow in a poignant show of frustration with the ex-KGB spy's decision to seek a third presidential term after dominating Russia for 12 years.