Alexei Navalny: Russian jail term is condemned
Protesters were met with a large police presence in Moscow
Profile: Alexei Navalny
Navalny: A Mandela moment?
Fates of Putin's enemies
Profile: Vladimir Putin
The conviction and jailing of Russian protest leader Alexei Navalny has led to widespread criticism.
Navalny was imprisoned for five years for embezzlement from a timber firm. He had denied the charges, saying the trial was politically motivated.
The EU said the verdict posed "serious questions" about Russian law, while the US said it was "deeply disappointed".
Later, police detained dozens of protesters following scuffles in Moscow, St Petersburg and other cities.
Thousands took to the streets for protests which continued late into the evening.
In Kirov, where the trial was held, at least two Navalny supporters were arrested after the verdict and sentence were announced.
Bridget KendallDiplomatic correspondent, BBC News
In the last year or so Alexei Navalny had become probably Vladimir Putin's most prominent critic.
And his apparent sympathy for nationalist sentiments, reflected in his infamous jibes at the expense of non-Russian migrants, while upsetting liberals, would have worried the Kremlin.
Not only is he young and vigorous and an effective user of social media, he is also a talented orator. If ever he gained access to mainstream Russian media - and crucially, Russian state television - he could appeal to the same strata of society Mr Putin needs to maintain his hold on Russian politics.
One can be sure that Navalny, who made his reputation through whistle-blowing activities as a lawyer, will use every avenue to appeal against his sentence. Even if he is unsuccessful, a life behind bars will not entirely silence him. The activities of Mikhail Khodorkovsky, an earlier political opponent of the Kremlin who ended up with two long sentences, have shown that in today's Russia it is still possible to be heard from inside prison.
But once the appeals are exhausted, today's verdict bars Navalny from political office. He has already withdrawn from the race to become mayor of Moscow. The Kremlin will hope the outcome of this trial will in effect sideline him from the frontline of Russian politics.
'Parody of a trial'
The 37-year-old is a leading campaigner against President Vladimir Putin's United Russia party, and regularly blogged about corruption allegations.
He came to public attention when he inspired mass protests against the Kremlin and President Putin in December 2011.
Before he was handcuffed and led away, Navalny urged his supporters to continue his anti-corruption struggle, tweeting: "Don't sit around doing nothing."
Later it emerged that he could be temporarily freed until the sentence comes into force in 10 days' time, or pending an appeal.
Navalny had recently registered his candidacy for the next mayor of Moscow, and Interfax news agency quotes the head of his election headquarters as saying he will take part in the September poll if he remains free.
But Russian officials say if the verdict does come into force, he will be barred from running in any future election.
Navalny was found guilty of heading a group that embezzled timber worth 16m rubles ($500,000; £330,000) from the Kirovles state timber company while working as an adviser to Kirov's governor Nikita Belykh.
The prosecution had asked for a six-year jail sentence, but judge Sergei Blinov decided on five years, and said there were no extenuating circumstances that would warrant keeping Navalny out of prison.
Navalny's co-accused, Pyotr Ofitserov, was also found guilty, and given a four-year jail sentence.
State television has only shown limited interest in the process despite Navalny's prominence, but online the trial has been followed extensively.
The BBC's Moscow correspondent Daniel Sandford says that when the sentence was handed down, there were tears from Navalny's supporters and an explosion of anger on the social networking sites that he has used so effectively.
Anti-Putin activist and former cabinet minister Boris Nemtsov told reporters the trial was "completely fabricated from start to finish, and even the judge could not say what the reason for the crime was".
Jailed former oil executive and Putin critic Mikhail Khodorkovsky said the conviction was "predictable and unavoidable", according to the independent Ekho Moskvy website.
"There is nothing unusual for the government's opponents to be convicted of crimes in Russia," he added.
Alexei Navalny (centre) took a 12-hour train journey to Kirov for the verdict.
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The last Soviet leader, Mikhail Gorbachev, said using courts to punish political opponents was "unacceptable".
In a statement posted on his charity's website, he said: "Everything I know about this case... unfortunately confirms we do not have independent courts."
The US ambassador to Moscow, Michael McFaul, said: "We are deeply disappointed in the conviction of Navalny and the apparent political motivations in this trial."
A spokesman for the EU's top foreign policy official, Catherine Ashton, said the embezzlement charges were unsubstantiated, and that Navalny's jailing posed "serious questions as to the state of the rule of law in Russia".
Alexei Navalny's rise to prominence
2008: Started blogging about allegations of corruption at some of Russia's big state-controlled firms
Nov 2011: Ahead of parliamentary poll, he criticised President Putin's United Russia, famously dubbing it the "party of crooks and thieves"
Dec 2011: After the poll, he inspired mass protests against the Kremlin, and was arrested and imprisoned for 15 days
Oct 2012: Won most votes in a poll to choose opposition leadership
April 2013: Went on trial
July 2013: Declared himself a candidate for Moscow mayoral election
July 2013: Found guilty of theft and embezzlement
Profile: Alexei Navalny
A thorn in Putin's side
A spokesman for German Chancellor Angela Merkel said the trial had "raised doubts about whether criminal justice was the main motive".
"Five years in prison appears disproportionate, given the alleged crime," Steffen Seibert added.
Russian rights group Memorial said the country "now has one more political prisoner", while Amnesty International's Europe and Central Asia director, John Dalhuisen, said in a statement: "This was a parody of a prosecution and a parody of a trial."
But the Kremlin denies that Mr Putin uses the courts for political ends, and the judge rejected Navalny's claim the trial was politically motivated.
Pro-government analyst Sergei Markov said many Navalny supporters "realise deep in their souls that the court has proven in a normal way that Navalny was a thief".
Our correspondent said Navalny smiled in a resigned manner when the guilty verdict came.
His wife, Yulia Navalnaya, said her husband knew he would get a custodial sentence and was mentally prepared to go to prison.