Russian riot police have detained hundreds of protesters at rallies challenging Vladimir Putin's presidential election victory.
The arrests came as a Downing Street spokesman said British Prime Minister David Cameron called Mr Putin to say he looked forward to working with him to overcome obstacles between the two countries.
"The leaders agreed on the importance of building a stronger relationship, without disguising differences and areas of concern," the spokesman said.
In Moscow, officers detained opposition figurehead Alexei Navalny along with fellow opposition leaders Sergei Udaltsov and Ilya Yashin.
"They robbed us," Mr Navalny, an anti-corruption blogger, told a crowd of protesters in the capital's Pushkin Square.
Hundreds had refused to leave the square at the end of the demonstration, which opposition leaders said was attended by up to 20,000 people.
Between 500-1,000 people were arrested, the opposition leaders said - but police put the number at 250 and said 14,000 people had attended the rally.
"We are the power," said Mr Navalny to cheers, before being taken away by officers.
Mr Yashin told independent Dozhd TV from a police van by phone: "People were calm, but at some point police started moving in, kicking people, tearing their clothes.
"There was no aggression on our part."
Earlier, police arrested Eduard Limonov, the leader of the Other Russia movement, and about 50 of his supporters in Moscow after they held an "unsanctioned protest".
In Russia's second city of St Petersburg, authorities reportedly arrested 300 protesters, including local deputies from the liberal Yabloko party.
Opposition leaders have accused the Russian authorities of widespread fraud and claim many people were allowed to cast multiple votes.
But, speaking at a rally attended by tens of thousands of supporters outside the Kremlin in Moscow, an apparently tearful Mr Putin insisted it had been an "open and honest" contest.
The victory party began the moment the polls closed despite the vote not being fully counted.
In freezing conditions, a crowd of around 100,000 filled the streets in the heart of the capital, waving Russian flags and cheering for their new president.
Mr Putin's spokesman Dmitry Peskov claimed a freezing wind was responsible for the tears, but there was clearly strong emotion behind the new president's words.
Later, via webcam, he thanked workers in a remote region saying they had managed to defeat those who had "insulted the working man", presumably referring to the protest movement.
The 59-year-old former KGB spy picked up almost 64% of the vote - enough to avoid a run-off against the second-placed candidate. Independent monitoring website Golos reported thousands of allegations of cheating, putting his actual figure at 50%.
The claims of so-called "carousel voting" were being received by independent monitoring group Golos , according to spokeswoman Lilia Shibanova.
Golos said it had registered at least 3,000 reports of violations nationwide - but an interior ministry spokesman insisted there had been no major violations.
The alleged fraud came despite the presence of thousands of independent observers and webcams at polling stations.
Mr Putin's campaign chief Stanislav Govorukhin insisted: "This is the cleanest election in Russia's entire history.
"The violations our rivals and the opponents of our president will now speak of are laughable."
Mr Putin, who was president between 2000 and 2008, is now set for a new six-year term as leader. He has been serving as prime minister for the past four years.
Communist leader Gennady Zyuganov, who came second with 17%, said his party would not recognise the official results of the election, calling it "illegitimate, dishonest and untransparent".
He added that with public anger increasing Mr Putin "would not be able to rule like he used to".
The central election commission said the turnout was 64%, almost 6% down on the 2008 presidential vote but higher than for the parliamentary elections in December.
Mr Putin's opponents vowed to step up their protests, the biggest in Russia since he first gained power 12 years ago.
Police - who have brought in 6,300 extra officers from across Russia - have warned they will break up any unauthorised gatherings.
There were reports of widespread voting violations during the election, including allegations people were being bussed around to cast their ballots several times.
The election was held against a backdrop of popular discontent, sparked by allegations of widespread fraud during December's parliamentary elections in favour of the United Russia party.
Mr Putin has remained the dominant force in Russian politics since being forced to step aside as president because was barred from a third straight term by the constitution.
But many voters have grown tired of his macho antics - such as horse riding bare-chested - and his popularity has waned amid falling living standards despite Russia's huge natural resources.
He will begin his third term as president in May.
:: Meanwhile, outgoing president Dmitry Medvedev ordered a review of the convictions of jailed oil billionaire Mikhail Khodorkovsky, whose imprisonment critics have said was politically motivated, and 31 other people.
The order follows Mr Medvedev's meeting in February with protest leaders. Russian news reports say leaders at that meeting presented him with a list of names of those they regarded as political prisoners.