Police said the meeting in central Samara gathered about 70 people instead of the 1,000 declared earlier, and 150 journalists and passersby. The opposition's Web site said the number of protesters was 500.
Russian media said the march did not start for a while because there were so few protesters that they could not organize themselves into columns. The marchers are now trying to start a meeting on the embankment.
The opposition said surrounding streets had been cordoned off by police, preventing others from joining the march.
Western media reported Friday that Russian police had prevented Garry Kasparov, the leader of the United Civil Front opposition movement and a former world chess champion, from boarding a plane to Samara along with the other two opposition leaders.
An official spokesman for the police department overseeing the airport confirmed the detention. "We had suspicions that their tickets were fake," he said, adding that opposition leaders had been released and could have boarded later flights if they wanted to.
