Russia's foreign ministry has dismissed US criticism of its parliamentary election as "unacceptable".
Washington had voiced "serious concerns" over the conduct of Sunday's vote, in which
prime minister Vladimir Putin's ruling United Russia party suffered a decline in support but won
a slim majority in the lower house.
Several thousand people protested in central Moscow against what they said was a fraudulent parliamentary election, shouting "Revolution!" and calling for an end to Mr Putin's rule.
The protest, a day after an election in which Russians sharply reduced the parliamentary majority held by Mr Putin's United Russia party, appeared to be one of the biggest opposition demonstrations in Russia in years.
Protesters initially gathered in the rain on a tree-lined boulevard where they had permission for a rally. They denounced the vote as shameful and shouted "Russia without Putin!"
Many protesters then tried to march out onto a major street and were confronted by a chain of police officers who locked elbows and divided the surging crowd into two parts. Several people were detained in the area.
Several dozen protesters managed to break through or circle around the police and marched toward the Kremlin. Police detained at least 30 near the Kremlin and bundled them into buses.
Mr Putin's spokesman said today any unsanctioned street protests must be stopped.
"Those who hold sanctioned demonstrations should not have their rights limited in any way - and that is what we are observing now," Dmitry Peskov said the day after several thousand Russians protested against the result of Sunday's parliamentary election, which they said was rigged.
"The actions of those who hold unsanctioned demonstrations must be stopped in the appropriate way," he said.
US secretary of state Hillary Clinton suggested Russia's elections were neither free nor fair as she made a broad plea today for digital freedoms at a European security gathering.
Speaking to ministers of the Organisation for Security and Cooperation in Europe (OSCE), she also accused Belarus of "unremitting persecution" of its opposition and suggested Ukraine prosecuted ex-premier Yulia Tymoshenko for political reasons.
For a second day running, Ms Clinton cited "serious concerns" about the elections in which Russian prime minister Vladimir Putin's parliamentary majority was slashed in a vote marred by accusations of ballot-stuffing and other irregularities.
"When authorities fail to prosecute those who attack people for exercising their rights or exposing abuses, they subvert justice and undermine the people's confidence in their governments," Ms Clinton said in a speech at the meeting of the 56-nation OSCE, Europe's biggest rights watchdog.
"As we have seen in many places, and most recently in the Duma elections in Russia, elections that are neither free nor fair have the same effect," she added, in comments that went a step further than her criticism of the vote yesterday.
Ms Clinton said two former presidential candidates in Belarus remain imprisoned a year after a government crackdown and she voiced concern about the case of Ukraine's Tymoshenko, who faces seven years in prison for abuse of office.
"There are growing restrictions on the exercise of fundamental rights through the OSCE region," she said.
Speaking today, Mr Putin said the government needed changes after the election.
"There will of course be a significant renewal of personnel in the government," he told members of his ruling United Russia party.
After the biggest electoral upset of his 12-year rule, Mr Putin said he would adjust to society's demands for modernisation.
Reuters