Russian opposition leader Alexei Navalny said he was detained by police Friday after leaving his home in Moscow to speak at a pre-election rally.

Navalny tweeted Friday morning, in Russian, saying he had been “detained and was being taken somewhere,” but could not say where.

He said the police wanted to talk to him about something, but the reason was not disclosed. It remained unclear whether he would be charged with any crime.

Russia is set to hold an election in March, with President Vladimir Putin expected to seek another term.

Navalny said he was told by the country’s election commission that he would not be allowed to run for president, but he insisted he would stand against Putin either way, Reuters reported.

The activist was due to speak at pre-election opposition rally in Nizhny Novgorod on Friday.

Navalny has been detained three times this year. In June, he spent 30 days in jail after police arrested him at an anti-corruption protest in Moscow.

Navalny is among the most popular opposition leaders in Russia, known for highlighting the Kremlin’s corruption with viral investigations.

Yesterday, he published an investigation into Russian state media journalist Vladimir Solovyov – described as one of Putin’s “frontline soldiers,” the Moscow Times reported.