Thousands of people gathered in downtown Moscow to protest the May 7 inauguration of Vladimir Putin, who is to return to the Kremlin after four years as prime minister.

“Putin is a thief!” chanted protesters at the beginning of the march, which leads from near Gorky Park to a square a short walk from the Kremlin. A police helicopter hovered above marchers.

Police said there were 8,000 people at the so-called March of Millions rally.

March organizer Sergei Udaltsov, leader of the Left Front movement, said in a Twitter post that there were “no less than 100,000” at the rally. Other opposition figures, including former world chess champion Garry Kasparov, estimated some 20,000 protesters. The march is to conclude with a rally on Bolotnaya Square, the site of two previous mass opposition rallies last winter. Udaltsov said police in several Russia regions had detained activists attempting to travel to the demonstration.

He also accused police of attempting to disrupt the rally by refusing to allow equipment for the stage to be brought onto the square.

Meanwhile, Putin supporters gathered for a separate rally. Organizers said some 50,000 people were in attendance. Opposition figures accused the authorities of coercing government employees to attend previous pro-Putin rallies in recent months.

Putin admitted earlier this year that the accusations could be true, but said the effect on crowd numbers should "not be exaggerated."