People gather in a square during a rally, held by supporters of Russian opposition leader Alexei Navalny for a boycott of a March 18 presidential election, in Moscow, Russia January 28, 2018. REUTERS/Tatyana Makeyeva

MOSCOW (Reuters) - Russian opposition leader Alexei Navalny was detained soon after joining a rally in Moscow on Sunday, the anti-corruption activist wrote on Twitter.

Hundreds of his supporters joined a nationwide day of protest against the authorities on Sunday, calling on voters to boycott what they said would be a rigged presidential election on March 18.