Yulia Navalnaya said on Monday she would continue the fight of her dead husband Alexei Navalny for a "free Russia" and called on opposition supporters to battle President Vladimir Putin with greater fury than ever.

Navalnaya's call from abroad for resistance to Putin comes less than a month before a presidential election that is almost certain to hand the Kremlin chief another six-year term.

In a nine-minute video message laced with rage, Navalnaya, 47, said Putin had killed her husband and in doing so had cut away half her heart and robbed their two children of a father.