MOSCOW (AP) -- One by one, thousands of mourners and dignitaries filed past the white-lined coffin of slain Kremlin critic Boris Nemtsov on Tuesday, many offering flowers as they paid their last respects to one of the most prominent figures of Russia's beleaguered opposition.
So many came, that when the viewing ended after its scheduled four hours, a line of people hundreds of meters (yards) long still waited outside the Sakharov Center, named after the Soviet-era dissident and Nobel Peace Prize laureate Andrei Sakharov.
Nemtsov was shot to death late Friday while walking on a bridge near the Kremlin with a companion. No suspects have been arrested.
The killing has deeply shaken Russia's small and marginalized opposition movement. Many opposition supporters suspect the killing was ordered by the Kremlin in retaliation for Nemtsov's ardent criticism of President Vladimir Putin. Authorities have suggested several possible motives, including a provocation aimed at tarnishing Putin's image.
Those who filed by ranged from committed opposition activists to ordinary citizens young and old.
"He was our ray of light. With his help, I think Russia would have risen up and become a strong country. It is the dream of all progressive people in Russia," said 80-year-old Valentina Gorbatova.
"I am here to show that aside from the 80 percent of Russians who don't watch anything but state television and don't think for themselves, there are ... us, who do think and see that the government system is unfair and that we need to change a lot in our country," said Marsel Shamsudinov, who had come from the city of Kazan, 700 kilometers (450 miles) to the east to pay his respects.
Alexei Navalny, Russia's leading opposition figure, was serving a short jail term for unauthorized leaflet distribution and was not allowed to attend. Some European officials from Poland and Latvia were not allowed to enter Russia for the ceremony.
Nemtsov, 55, had been a deputy prime minister under President Boris Yeltsin and was widely seen as a rising young reformer. However, in the Putin era Nemtsov's party lost its seats in parliament.
Although his influence in mainstream politics vanished, Nemtsov remained visible as one of Putin's most vehement critics. In a radio interview a few hours before his death, he denounced Putin for his "mad, aggressive" policies in the Ukraine crisis.
After the viewing, his body was buried at a cemetery on Moscow's western edge, as relatives and about 100 bystanders looked on.
The cemetery's graves included those of three plotters of the failed 1991 coup attempt and that of Alexander Yakovlev, widely regarded as the architect of Glasnost, Soviet leader Mikhail Gorbachev's policy of greater openness.
Among those attending the viewing were U.S. Ambassador John Tefft and former prime minister Mikhail Kasyanov, who has gone over to the opposition. Russian deputy prime ministers Sergei Prikhodko and Arkady Dvorkovich and Yeltsin's widow Naina also came, along with tycoon and New Jersey Nets owner Mikhail Prokhorov, who ran against Putin in the 2012 presidential race.
"They probably know that if they don't come, then at some point people will be coming for them," Irina Khakamada, co-leader of a liberal party in parliament with Nemtsov, said of the Russian officials' presence.
Veteran human rights activist Lev Ponomarev, echoing the view of many opposition figures, said the strong nationalism and intolerance of dissent that has risen up under Putin and is on display on Russian state-controlled television has coarsened society and encouraged violence.
"In this atmosphere of violence and hate, these killings will only continue," he said.
Russian officials have promised a thorough investigation of the killing, but skepticism remains strong.
The Ukrainian woman who was walking with Nemtsov when he was shot, Anna Duritskaya, has returned to her homeland after several days of questioning, said Vladimir Markin, a spokesman for Russia's Investigative Committee.
Russian media have made much of Duritskaya's presence, noting she was three decades younger than Nemtsov and reportedly an aspiring model.
Many commentators said that, like other key opposition leaders, Nemtsov was constantly being shadowed by police, so it's hard to imagine that his killing could go unnoticed by them. Some noted that Nemtsov died on the newly established holiday commemorating the Special Operations Forces, honoring troops who swept through Ukraine's Crimean Peninsula, setting the stage for its annexation by Russia a year ago.
Nemtsov's killing was the biggest political assassination in Russia since 2006, when another Kremlin foe, journalist Anna Politkovskaya, was shot to death in the elevator of her Moscow apartment building on Putin's birthday. Five Chechens were convicted in the case last year, but it has remained unclear who ordered the killing.
Some observers speculated that certain members of a hawkish, isolationist wing of the government could have had a hand in Nemtsov's death, possibly hoping to provoke outrage abroad and further strain Russia's ties with the West.
Those relations already are at their lowest point since the Cold War because of the fighting in eastern Ukraine. The government in Kiev blames Russia for supporting and arming the separatists. Russia denies the charge but NATO says it has satellite photos of Russian military equipment in eastern Ukraine.