Shibanova said some of the worst violation were in the Volga River city of Samara, where observers and election commission members from opposition parties were barred from verifying that the ballot boxes were properly sealed at all polling stations.
Social media were flooded with messages reporting violations. Many people reported seeing buses deliver groups of people to polling stations, with some of the buses carrying young men who looked like football fans.
In Moscow, several journalists, including a photographer for The Associated Press, were briefly detained after taking pictures at a polling station.
Mikhail Kasyanov, a former prime minister during Putin's first presidential term, said he and other opposition activists who voted Sunday are under no illusion that their votes will be counted fairly.
"It is absolutely clear there will be no real count," he said. "The authorities created an imitation of a very important institution whose name is free election, that is not free and is not elections."
In a number of Russian regions, the official results differed sharply from the exit polls, with United Russia doing far better than the polls indicated.
A few dozen activists of the Left Front opposition group tried to stage a protest just outside Red Square on Sunday, but were quickly dispersed by police, who detained about a dozen of them. Later in the evening, police said they arrested more than 100 other opposition demonstrators at another Moscow square and about 70 in St. Petersburg.
The websites of Golos and Ekho Moskvy, a prominent, independent-minded radio station, were down on Sunday. Both said the failures were due to denial-of-service hacker attacks. Ekho Moskvy's site came back up after the polls closed.
Golos, which is funded by U.S. and European grants, has come under heavy official pressure in the past week after Putin accused Western governments of trying to influence the election and likened recipients of Western aid to Judas.
Shibanova, the Golos leader, said its hotline was flooded Sunday with automated calls that effectively blocked it. Prior to the vote, many of the group's activists were visited by security agents, while Shibanova was held for 12 hours at an airport and forced to hand over her laptop.
The group had compiled some 5,300 complaints of election-law violations ahead of the vote, most of which were linked to United Russia. Roughly a third of the complainants — mostly government workers and students — said their employers and professors were pressuring them to vote for the party.