The protester interrupted a newscast, holding up a sign calling on people not to believe Russian propaganda.
Police detain more than 3,000 in Moscow and other cities, says a monitoring group, as tens of thousands of supporters of jailed Kremlin critic take to streets following his call to protest against President Vladimir Putin's rule.
Demonstrators were rallying against the exclusion of several opposition candidates from the ballot for this fall's Moscow council elections.
Alexei Navalny was picked up by police before he was due to head off to an unsanctioned protest. Electricity to his office was also reportedly cut off.
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