The two activists, one of which works for independent election watchdog Golos, were campaigning as part of "Strategy 31," which supports the section of the constitution that provides for the right to public assembly.
Svetlana Zelenova, a witness at the scene, told Interfax that two police vans arrived at the square and "in a pretty rough manner" threw the protesters into one van and the tents in the second.
Police are yet to comment on the arrests.
Nine Nizhny Novgorod activists were sentenced last week to between one and 10 days in jail for participating in a March 10 unsanctioned march against vote falsifications.
Opposition leaders in Moscow, including Left Front head Sergei Udaltsov and blogger Alexei Navalny, have spoken publicly about wanting to set up a tent city in the capital as a means of exerting pressure on the government to accede to their demands for reform. Moscow Mayor Sergei Sobyanin has said the city will not allow demonstrators to establish such an outpost, while pro-Kremlin commentators have warned of the dangers of a movement similar to Ukraine's 2004 Orange Revolution, during which protesters of election fraud camped out for weeks in Kiev's Maidan Square.