Leading Russian opposition activist Alexei Navalny says he believes President Vladimir Putin was responsible for his poisoning.

Russian doctors say leading opposition figure Alexei Navalny - who supporters believe was poisoned - remains too ill to be moved to Germany for treatment.

More than 15,000 Russians on Sunday endorsed the candidacy of Alexei Navalny, seen as the only Russian opposition leader who stands a fighting chance of challenging strongman Vladimir Putin in a March vote.

Police in Moscow detained more than 20 anti-corruption protesters who took to the streets on Sunday in a follow-up of last week's large-scale demonstrations in the Russian capital, according to a Reuters witness.