Hundreds of opposition activists joined several thousand Communists in a Victory Day march down Tverskaya Ulitsa that was also a show of resilience for the "non-stop" protest against President Vladimir Putin's inauguration.
The protesters, many wearing the opposition movement's emblematic white ribbon, denounced Putin as a thief and called for his resignation.
They stood out from the majority of marchers by their relative youth and aggressive slogans, prompting curious looks from marchers with the mainstream Communists.
Police estimated the entire crowd at 3,500, according to a statement on the their website.
Protesters have now appeared for three straight days, beginning with massive and unprecedentedly violent demonstration on Bolotnaya Ploshchad and continuing in a smaller, peripatetic protest that moves from square to square to avoid detentions by ever-present police.
Their appearance on Wednesday could signal growing solidarity between the protest movement and the Communist Party, the country's most powerful political force after the pro-Kremlin United Russia.
Left-wing forces are prominent in the opposition movement. One of its most visible leaders, Sergei Udaltsov, is an avowed Stalinist whom many believe could eventually lead the Communists.
But while Communists interviewed by The Moscow Times said they shared the opposition's distaste for Putin, who was inaugurated on Monday, they expressed suspicion about some of its most prominent faces.
"Kasyanov, Nemtsov, Ryzhkov — they're no different from Putin," said Yury Gorbach, 47, a worker at Metrovagonmash, which manufactures metro train cars. "They just want power."
Wednesday's march ended with a rally on Lubyanskaya Ploshchad in front of the headquarters of the Federal Security Service amid reports that opposition activists had begun gathering in Alxeander's Garden, by the Kremlin walls, and at Chistiye Prudy, the site of earlier gatherings.