About 10,000 Russians joined a march Tuesday against President Vladimir Putin's third Kremlin term, city police said, although organisers put the figure far higher.
The Moscow police department said "up to 10,000" had set off on a demonstrations at 1.00 pm (0900 GMT) that was due to conclude with a mass rally against Putin's rule.
Organisers put the figure at about 20,000 and said it was likely to grow.
The march will take protesters down Moscow's Boulevard Ring toward Sakharov Avenue, scene of a dramatic demonstration last December against the outcome of disputed parliamentary elections that month.
City authorities allowed up to 50,000 to take part in Tuesday's event, which coincides with the patriotic Russia Day holiday marking the country's 1990 declaration of independence from Soviet rule.
Moscow police said they were sending 12,000 riot officers and interior ministry troops onto the streets of the capital to keep order.
Leftist radical Sergei Udaltsov for his part estimated the early turnout at 50,000, the ITAR-TASS news agency reported.