MOSCOW (AP) — Thousands were converging Sunday in central Moscow to mourn veteran liberal politician Boris Nemtsov, whose killing on the streets of the capital has shaken Russia's beleaguered opposition.

President Vladimir Putin has marginalized and intimidated his political opponents, jailing some and driving others into exile. Nemtsov, 55, was among the few prominent opposition figures who had refused to be cowed.

The mourners are to march to the bridge near the Kremlin where Nemtsov was gunned down shortly before midnight Friday. The march could serve to energize the opposition or it could prove to be a brief outpouring of emotions that once again dissipates in a climate of fear.

Whoever was responsible for the slaying, the signal it sends to Putin's foes is that if Nemtsov can be killed for his political activism then no one is safe. As a former deputy prime minister and longtime politician, he retained strong ties among Russia's political and business elite.

Russia's federal investigative agency said it was looking into several possible motives for his killing.

The first possibility, the Investigative Committee said, was that the murder was aimed at destabilizing the political situation in Russia and Nemtsov was a "sacrificial victim for those who do not shun any method for achieving their political goals."

This suggestion echoed comments by Putin's spokesman and other Russian politicians that the attack was a "provocation" against the state.

Opposition activists had planned a protest rally on Sunday, which the city demanded they hold in a suburban neighborhood. After Nemtsov's death, they called instead for a demonstration to mourn him in central Moscow. The city gave its quick approval.