The United States has voiced concern over the treatment of protesters rounded up in Russia amid major opposition demonstrations to contest the results of parliamentary elections.

"We've expressed our concerns about the treatment of all those being arrested who were exercising their rights to peaceful protest," State Department spokesman Mark Toner told reporters on Wednesday.

"We would obviously support the rights of anyone to peaceful protest -- emphasis on peaceful -- anywhere in the world," Toner said. "Russia's no different."

Toner said he was not aware of any formal diplomatic exchange about the protests. But Secretary of State Hillary Clinton has publicly questioned the fairness of the election, angering Russia.

Riot police in helmets dragged over 550 protesters into detention vans Tuesday evening in central Moscow, during what commentators have described as the biggest opposition rallies since the chaotic early 1990s.

Despite the arrests, the opposition has pledged more rallies to protest what it sees as fraud in Sunday's election, in which Prime Minister Vladimir Putin's United Russia party won, but with a reduced majority.

Several leading US senators criticised the arrests and the "blatant fraud" of the election in a joint statement.

"We condemn the sweeping arrests of hundreds of opposition leaders, journalists, and human rights activists in Russia and the use of violence against peaceful protesters," Republican John McCain, Independent Joseph Lieberman and Democrat Jeanne Shaheen wrote.

"We call upon the Russian government to release all of the political prisoners it has unjustly detained and clarify the whereabouts and condition of those individuals in its custody," they added.

Prime Minister Vladimir Putin's United Russia party won the polls, but with a sharply reduced majority, an indication his popularity might be waning.

International observers said the polls were slanted in favor of United Russia, and the opposition says that the ruling party's performance would have been even worse in free polls.