The leader of the Russian opposition, and one of the most vocal critics of Vladimir Putin, Alexei Nalavny, has been arrested for violating the conditions of his house arrest, just hours after his sentencing.
The opposition activist - who ran for mayor of Moscow in 2013 - had been convicted of fraud alongside his brother Oleg earlier on Tuesday.
Although Alexei Navalny received a three-and-a-half-year suspended sentence, Oleg Navalny, a father-of-two who has no role in the opposition movement, was jailed for the same period.
Navalny said on Twitter that he planned to join protesters in Moscow and posted a picture of himself on what appeared to be a metro train.
"I may be under house arrest, but today I really want to be with you. That’s why I am going too," he wrote.
The blogger's supporters have called for a mass protest near the Kremlin, with at least 18,000 pledging on Facebook to attend.
Moscow officials warned that "all unsanctioned actions will be prevented by the security forces".
TV footage showed several thousand anti-Kremlin demonstrators gathered in the dark near Red Square and ringed by police or soldiers with riot shields.
Navalny tweeted shortly after being taken into custody as he approached the rally, urging others to stay and protest.
"I was detained, but they won't be able to detain everyone," he wrote.
The verdict in the brothers' case was scheduled for next month, but was abruptly moved forward to the day before New Year's Eve, the main holiday in Russia, leading to speculation that authorities wanted to head off planned protests.
The trial - which saw the pair accused of stealing 30 million roubles, around $500,000 (£372,000) at the current exchange rate, from two firms - was viewed by many critics as part of a campaign to stifle dissent.
Alexei Navalny reacted angrily to the jailing of his brother, shouting out: "Aren't you ashamed of what you're doing? You want to punish me even harder?"
He briefly entered the metal cage that his brother was put into after the verdict and appeared to be holding back tears.
The European Union said the verdict appeared to be "politically motivated" but called for protesters to show restraint.
