A stream of people arrived for a memorial service taking place at the Sakharov Center.
Some held flowers as they walked past the open casket of the prominent Kremlin critic and paid respects to his mother, Dina Eydman, and his children Anton and Zhanna.
Hundreds more lined up in the cold outside.
The Investigative Committee of the Russian Federation is offering a reward of 3 million rubles ($49,000) for information leading to the arrest of the killer. It said the tipster will be guaranteed anonymity.
CNN’s Alla Eshchenko, Holly Yan and Emma Burrows contributed to this report.