A month ahead of presidential elections, thousands of Russians rallied in the capital city of Moscow Sunday in honor of Kremlin critic Boris Nemtsov, who was murdered on this day three years ago.

In a rare sanctioned opposition gathering in Russia’s capital, many carried flags, portraits of Nemtsov, placards and flowers in frigid temperatures as low as minus 14 degrees Celsius.

Moscow police, who are often accused of underestimating opposition crowd sizes, said that 4,500 people attended the rally. Pro-opposition monitors said the figure was over 7,000.

Former presidential candidate Alexey Navalny, an anti-corruption campaigner who has been blocked from participating in the elections over legal problems widely seen as manufactured to keep him out of the race, was reported to have been in attendance.

Nemtsov, one of Russian president Putin’s most vocal critics, was shot in the back late at night while walking across a bridge just meters from the Kremlin in 2015. He was working on a report examining Russia’s role in the conflict in Ukraine at the time of his death.

Last year, a Russian court sentenced Saur Dadayev to 20 years in prison and four accomplices between 11 and 19 years. Dadayev initially pleaded guilty, but later recanted, saying he was tortured into the confession.

While the verdicts were welcomed by supporters of Nemtsov, the investigation and trial were condemned for failing to uncover the masterminds of the killing or addressing the motive, which is widely believed to be political.