"We were simply demanding that the laws of this country be adhered to," said group spokesman Alexander Averin.
He said that 70 activists had turned up and shouted slogans outside the Duma building.
The Constitution states that parliamentary sessions are open to the public except in special cases.
"The police would not let us inside, but we tried anyway, and they hauled about 50 of us off to the police station," Averin said.
Monday's incident closely resembled last week's arrest of more than 30 activists who broke into the Finance Ministry to protest the government's economic policies.
In early September, 30 activists held another unsanctioned protest outside the Duma after they were denied entry to an open session.
"That did play a part" in Monday's protest, Averin said. "Why have all these laws if no one is going to follow them?"
Averin said the group was also protesting the government's treatment of citizens who lost their savings in the economic turmoil of the 1990s.
"We had to stop them before they entered the building," city police spokesman Yevgeny Gildeyev said. Duma security officials called police around 11:20 a.m., he said. The activists were charged with holding an unsanctioned protest.
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