MOSCOW (AP) — Thousands of people marched on a central Moscow boulevard Saturday in remembrance of a Russian opposition leader who was gunned down five years ago while walking on a bridge adjacent to the Kremlin.

The march also protested proposed changes to the Russian Constitution that opponents suspect are intended to give President Vladimir Putin a way to retain power once his term ends in 2024.

Protests carried placards with sayings such as “No to eternal Putin” and “No to the usurpation of power.”

Police said about 10,000 people took part in the march; a nongovernmental organization that counts attendance at political rallies estimated a turnout of 22,000.

Boris Nemtsov, one of Putin's most charismatic and energetic opponents, was shot to death on Feb. 27, 2015 as he walked on the bridge over the Moscow River.

An officer in the security forces of Chechnya's Kremlin-backed leader was sentenced to 20 years in prison for firing the shots that killed Nemtsov. Four other men were sentenced to 11 to 19 years for involvement in the slaying, but who organized and ordered the assassination has not been determined.

Demonstrations in Nemtsov's memory also took place Saturday in other large cities across Russia, including St. Petersburg, where about 2,000 people gathered, and in Novosibirsk and Vladivostok.

The arrest-monitoring organization OVD-Info said five demonstrators were detained in St. Petersburg, but there was no immediate information on charges.

No detentions were reported in the Moscow march.