The United States said Monday it did not consider results of the elections in Belarus legitimate, as violence broke out after President Alexander Lukashenko was declared the winner by a landslide.

The Obama administration also warned that treatment meted out to protesters by the authorities was "disproportionate" and warned escalating tensions were putting Washington's relations with Minsk at risk.

"The United States concurs with the assessment of the OSCE (Organization for Security and Cooperation in Europe). We cannot consider the election results yesterday as legitimate," State Department spokesman Philip Crowley said.

At the White House, President Barack Obama's spokesman Robert Gibbs also said the results of the polls, announced by the Belarussian Central Election Commission, could not be seen as legitimate or free and fair.

The United States joined the European Union in condemning the crackdown on anti-Lukashenko protesters after hundreds of people were arrested in Minsk and riot police were used to quell demonstrations.

"The United States strongly condemns the actions that the government of Belarus has taken to undermine the democratic process and (the use of) disproportionate force against political activists, civil society representatives and journalists," said Gibbs in a written statement.

"We call for the immediate release of all presidential candidates and the hundreds of protesters who were detained on December 19 and 20."

Gibbs also raised concern about signs that the Internet had been disrupted and called on the Belarus government to ensure freedom of expression.

He noted that in a December 1, joint communique, it was made clear that the development of US-Belarus relations was contingent on the Belarus government's respect for human rights and democracy.

"The actions taken over the last 24 hours, however, are a clear step backwards on issues central to our relationship with Belarus," Gibbs said.

The episode put the Obama administration at odds with the government of Russia, at a time when it is striving to cement a "reset" of ties with Moscow.

President Dmitry Medvedev described the situation in Belarus, a former Soviet republic, as an "internal affair."

Lukashenko won Sunday's polls outright with nearly 80 percent of the vote after a huge turnout of over 90 percent, the central election commission said.

However, election monitors from the pan-European security body OSCE said the presidential polls indicated that Belarus "still has a considerable way to go" in meeting its commitments for free and fair elections.

It said that while the overall voting process was assessed as good, the process deteriorated significantly during the vote count itself.