"According to new data coming in from various parts of central Moscow, around 500 people were detained for illegally taking part in rallies that were not sanctioned by the authorities," Viktor Biryukov told RIA Novosti.
The largest of the illegal rallies took place near Arbat metro station, bringing together activists from various anti-immigration groups.
Biryukov said police detained activists only in extreme cases, and that most have already been released. There were no serious incidents, he added.
Russia has marked Unity Day each November 4 since 2005, following a 2004 law signed by then-president Vladimir Putin. The holiday commemorates the liberation of Moscow from Polish-Lithuanian occupiers in 1612.
According to various surveys, most Russians are not aware of the historical meaning of Unity Day. However, the event has become popular with nationalist and far-right movements.
In Russia's second city, St. Petersburg, 45 people were arrested at nationalist rallies, a local police source told RIA Novosti.
The Movement Against Illegal Immigration made use of official celebrations taking place on Sennaya Square to get past police and reach the Kazan Cathedral, where they held an illegal rally. Several were arrested.
The far-right group Slavic Union brought together 150 activists for a "Russian March" and rally at St. Petersburg's Chernyshevsky Garden. A group leader stood on a children's climbing frame read out a statement on "freeing the Slavic people," and was greeted by supporters with Nazi-style salutes.
The officially sanctioned Russian March went ahead peacefully in Moscow, with around 1,500 nationalists attending.
The event, organized by the People's Union and Russian Image, started with a march along the Moscow River and ended with a rally by Hotel Ukraine, but fell short of the predicted 5,000 participants.
At the rally, the leader of the People's Union laid out his vision for Russia: "Russia should be neither European, nor American. Russia should be Russian," Sergei Baburin said. "Much has already been done, and we are happy with the new course and success of Russia. But there is still much to be done."
More than 70 people have been killed and some 300 injured in ethnically motivated violence in Russia so far this year, the deputy head of a think tank dealing with the problem said in late September.
Russian media reported late last year that teenage ultra-nationalist gangs may have been responsible for up to 50 race-hate murders in Moscow in 2007.
