Demonstrations have begun in Moscow and other Russian cities against President Vladimir Putin in an opposition effort to mobilize millions of Kremlin critics around the country.
Security was tight ahead of the event, whose organizers received official permission for the march and accompanying rally.
Some 7,000 uniformed and plainclothes officers were deployed in downtown Moscow, where they removed vehicles along main roads that protesters using for the the main march, which began at noon local time.
The Interior Ministry put the number of participants in Moscow at around 5,000 at the scheduled start, according to RFE/RL's Russian Service, although other sources cited considerably higher attendance figures.
Opposition leaders had called for a "March of Millions" to demand social and political reforms, fair elections, and the right for government critics to make their case on state television.
"Our main slogan is 'End political repression,'" one of the opposition leaders, Sergei Udaltsov
Other demands include early parliamentary and presidential elections and more government support to working families and to low-income groups, he said.
WATCH RFE/RL Russian Service's live video feed of the demonstration as it takes place:
For the first time the Russian Communist Party -- the biggest opposition party in parliament -- was to be represented in the march.
However, billionaire tycoon Mikhail Prokhorov, who came third in the March 4 presidential election and took part in some previous protests, said he was staying away. He told media the march had no coherent program.
Russian authorities have tightened laws and regulations since Putin retook the presidency six months after heavily criticized Duma elections maintained the ruling United Russia party's majority.
Organizers had received permission from the Moscow mayor's office for the march on September 15 with 25,000 participants but expected up to 50,000 people -- setting the stage for possible confrontations with the police.
Thousands-strong protests are also expected in major regional centers, including the second city of Saint Petersburg and Yekaterinburg in the Urals.
The demonstrations, the first major antigovernment protest in three months, come one day after Russia's parliament expelled opposition politician Gennady Gudkov from its ranks and stripped him of immunity from criminal prosecution. Gudkov reportedly pledged to be among the marchers in Moscow.
Parliament members allied with Putin charge that Gudkov violated laws prohibiting elected officials from operating private businesses while in office.
Opposition leaders have said Gudkov's expulsion is part of a Putin campaign to remove government critics from power.
The September 15 activities are also the first mass action since the sentencing of three members of punk collective Pussy Riot to two years in prison for an anti-Putin protest in an Orthodox cathedral.
The tough sentence they were given has become a rallying cry for many in the opposition and in the West concerned with the direction that a fresh Putin presidency signals.
With additional reporting by dpa, AFP, Reuters, Interfax, and ITAR-TASS