Mourners are lining up outside a Moscow human rights center for the funeral of slain opposition leader Boris Nemtsov.

Nemtsov was shot to death late Friday while walking on a bridge near the Kremlin with a companion. No suspects have been arrested.

The killing has deeply shaken Russia's small and marginalized opposition movement. Many opposition supporters suspect the killing was ordered by the Kremlin in retaliation for his ardent criticism of President Vladimir Putin, while authorities have suggested several possible motives including a provocation aimed at tarnishing Putin's image.

Nemtsov's body on Tuesday lay in a coffin lined in white fabric in the Sakharov Center in central Moscow. The center is named after the late Soviet-era dissident and Nobel Peace Prize laureate Andrei Sakharov.