Russian police stormed into the home of popular video blogger Vyacheslav Maltsev and took him to Moscow for questioning on April 13.
Maltsev, who unsuccessfully ran for parliament last year, live-streamed the scene from his apartment in the central Russian city of Saratov as police knocked down his door. Maltsev typically attracts around 100,000 views for his daily video blogs.
Accompanied by riot police, an unidentified Russian investigator was filmed ordering Maltsev, 52, to get dressed and leave for questioning in Moscow.
The Parnas opposition party, which listed Maltsev as number two on its party ticket during the September parliamentary elections, said Maltsev suffered a heart attack before he was put on a plane to Moscow.
Russian news agencies said Maltsev was under medical surveillance and being questioned in connection with an investigation into an attack on a policeman at an opposition rally in Moscow last month.
Interfax reported that a New Opposition Movement official said Maltsev was being held at the Tverskoye police station with no access to lawyers.
The March 26 rally, one of the largest protests in Russia in years, was not sanctioned by authorities.
Meanwhile, four people were arrested on suspicion of attacking the policeman at the rally, and others were being questioned, the Russian Investigative Committee said on April 13.
It said the four men are Aleksandr Shpakov, Stanislav Zimovets, Yury Kuly, and Andrei Kosykh, all between 30 and 40 years old.
Interfax quoted the New Opposition Movement as saying that searches were also conducted at the residences of rally organizers Ivan Beletsky and Yury Gorsky in Moscow. Interfax said Beletsky and Gorsky were later released.
The anticorruption rally was called by opposition politician Aleksei Navalny following his report on alleged luxury properties controlled by Prime Minister Dmitry Medvedev through shadowy foundations.
Navalny called the probe into violence against police at the rally "fabricated."
The populist politician -- who is has said he will run for president next year -- has announced another round of protests for June 12.
Russian authorities have a history of launching high-profile prosecutions after major protests. A probe into violence at a May 6, 2012 rally led to dozens of people being prosecuted, including for alleged offenses such as "throwing a lemon."
Many of those targeted in the 2012 case spent several years in jail while others fled the country.
With reporting by AP, AFP, Interfax, and RIA Novosti