Thousands of Russians paid their respects Tuesday to slain opposition leader Boris Nemtsov at a memorial service for the former deputy prime minister who was murdered Friday by yet unidentified attackers. His murder is strongly suspected to be connected to his years of government criticism and opposition to Russian action in Ukraine.

According to authorities, Nemtsov was walking over the Great Moskvoretsky Bridgenear the Kremlin in Moscow with his girlfriend when a man rushed out of a white car behind him and shot him four times. His girlfriend, 23-year-old Anna Durytska, was questioned by police over the weekend but allowed to fly home to Kiev, Ukraine. She was not able to attend the memorial service. Durytska said she did not see who attacked Nemtsov because the murder happened so quickly and the attackers drove off before she could see the face of anyone involved.

Tuesday's crowd was smaller than the estimated 20,000 people who attended an anti-war rally Sunday planned by Nemtsov before his death. The march turned largely into a memorial rally with many marchers holding up photos of Nemtsov and praising his 25 years in contentious Russian politics. Nemtsov was Russia's first deputy prime minister under President Boris Yeltsin following the fall of the Soviet Union. Many thought he was in line to become president, but Vladimir Putin was given the nod instead. Nemtsov had been a vocal critic of Putin since then.

Nemtsov's line of work constantly put him in danger. He was attacked with ammonia while running for mayor of Sochi in 2009 and received dozens of death threats over the years. He expressed suspicion that Putin wanted him dead in the weeks leading up to his death. Below are some images from Tuesday's funeral in Moscow.

2015-03-03T072321Z_1277450199_GM1EB3316NT01_RTRMADP_3_RUSSIA-NEMTSOV-GIRLFRIEND People stand in a line to attend Nemtsov's service. Photo: Reuters/Maxim Shemetov 2015-03-03T094336Z_335539973_GM1EB331D4I01_RTRMADP_3_RUSSIA-NEMTSOV People stand in line to attend a memorial service before Nemtsov's funeral. Photo: Reuters/Maxim Shemetov 2015-03-03T074012Z_25806719_GM1EB33176401_RTRMADP_3_RUSSIA-NEMTSOV-GIRLFRIEND A visitor holds flowers and a book "Confessions of the Rebel" by Russian leading opposition figure Boris Nemtsov. Photo: Reuters/Maxim Shemetov 2015-03-03T081147Z_1_LYNXMPEB2209B_RTROPTP_4_RUSSIA-NEMTSOV-GIRLFRIEND A man reacts as he waits to attend Nemtsov's memorial service. Photo: Reuters/Maxim Shemetov 2015-03-03T093654Z_732346484_GM1EB331CVA01_RTRMADP_3_RUSSIA-NEMTSOV-GIRLFRIEND (1) Lit candles are placed on the ground as people gather to attend a memorial service before Nemtsov's funeral. Photo: Reuters/Maxim Shemetov 2015-03-03T084821Z_374311999_GM1EB331AJJ01_RTRMADP_3_RUSSIA-NEMTSOV Muscovites wait to pay their respects to Nemtsov. Photo: Reuters/Maxim Shemetov 2015-03-03T083126Z_759033778_GM1EB3319RX01_RTRMADP_3_RUSSIA-NEMTSOV Mourners, including Dina Eidman (L), Nemtsov's mother, and relatives and acquaintances, surround a coffin at the memorial service for the opposition leader. Photo: Reuters/Maxim Zmeyev 2015-03-03T091039Z_1567979708_GM1EB331B6N01_RTRMADP_3_RUSSIA-NEMTSOV Nemtsov's mother, Dina Eidman looks over her son's coffin at the memorial service before his funeral. Photo: Reuters/Maxim Shemetov 2015-03-03T132348Z_428091967_GM1EB331NBJ01_RTRMADP_3_RUSSIA-NEMTSOV Mourners lay flowers for Nemtsov. Photo: Reuters/Maxim Shemetov