Russia's presidential polls won by Vladimir Putin were an "insult to civil society" due to mass violations, an activist group fronted by celebrities said Wednesday, refusing to recognize the results.

"Due to widespread violations, we consider it impossible to recognize the results of the presidential elections," the League of Voters said in a statement after Putin's crushing victory in Sunday's polls.

A female suicide bomber killed five police officers on Tuesday when she blew herself up at a checkpoint in the Russian Caucasus region of Dagestan, the regional interior ministry said Wednesday.

Two more people were shot dead by unidentified gunmen in a raid on a police station in another Dagestan town, Kaspiysk, in a surge of violence just days after Vladimir Putin was elected president.

The United States on Monday urged Russia to conduct an "independent, credible" probe of election results that swept Vladimir Putin back to the Kremlin, amid widespread reports of irregularities.

But U.S. State Department spokeswoman Victoria Nuland, without mentioning Putin by name, also said the United States "looks forward to working with the president-elect after the results are certified and he is sworn in."

Russian police Monday arrested over 500 people protesting Vladimir Putin's crushing victory in elections that the opposition denounced as illegitimate and international monitors said were skewed.

Western powers reacted warily to the prospect of Putin returning in a May inauguration to the Kremlin for a six year term from his current job as premier, urging a full investigation into the reports of violations.

The European Union on Monday urged more international action on getting humanitarian aid into Syria and called for Russia to recognize the need for new Syrian leadership, as Paris urged Vladimir Putin to mark his return to the Russian presidency by dropping Moscow's support for the Syrian regime.

"We need to see some action on Syria from the international community in order to get humanitarian aid that's so desperately needed into Homs right now," EU foreign policy chief Catherine Ashton told reporters in Prague.

Russian election monitors Monday sorted through thousands of complaints of alleged fraud in presidential polls, as they complained of pressure and even beatings during their work.

"The situation is really difficult," said Grigory Melkonyants, deputy head of Golos monitoring group, which fielded thousands of volunteers and took 14,000 calls from voters complaining about perceived fraud.

Western powers greeted Vladimir Putin's presidential election victory with resignation Monday, while leaders in Asia and the Balkans openly congratulated the Russian strongman on his return to the Kremlin.

The European Union's chief diplomat Catherine Ashton said the bloc merely "took note" of the outcome of Sunday's vote while there were assurances from the EU's most powerful members that they wanted to work with Moscow.

China's President Hu Jintao on Monday congratulated Vladimir Putin on his victory in the Russian presidential elections, the foreign ministry said.

Putin won back the Russian presidency, which he held from 2000-2008 before his four-year stint as prime minister, after securing almost 64 percent of the vote in an election the opposition said was undermined by fraud.

Russian leader Vladimir Putin crushed his rivals in presidential elections with almost 64 percent of the vote, according to results published Monday based on an almost complete vote count.

Putin won 63.75 percent of the vote in Sunday's polls, well ahead of his nearest rival the Communist Party leader Gennady Zyuganov who won 17.19 percent, based on a count of the vote from 99.3 percent of polling stations.

Vladimir Putin, who has ruled Russia in an era where stability was restored but liberties curtailed, may have won Sunday's polls with ease but he is reclaiming the Kremlin in unpredictable new times.

Barely troubled by serious opposition and sitting pretty at the top of an authoritarian power structure, Putin was jolted by the mass protests that broke out in the aftermath of December's parliamentary elections.