Who has more to fear in Moscow?

MOSCOW -- 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 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.

President Vladimir Putin has vowed a government investigation to get to the bottom of Nemtsov's murder, but many opposition supporters suspect the killing was ordered by the Kremlin in retaliation for his ardent criticism of Putin.

And former CBS News Moscow bureau chief Beth Knobel says that, given the extent to which all of Russia's judiciary, law enforcement and intelligence agencies are controlled directly by the Kremlin, there is "no hope for a real, independent investigation."

Authorities have suggested several possible motives, including a provocation aimed at tarnishing Putin's image.

Nemtsov's body lay in a coffin in the Sakharov Center in central Moscow, named after the late Soviet-era dissident and Nobel Peace Prize laureate Andrei Sakharov.

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.

The funeral also has led to EU-Russian relations taking a further dip, when the European Union strongly condemned Moscow for banning Polish and Latvian lawmakers from entering the country to attend it.

European Parliament President Martin Schulz called the bans a "high affront" and said he will intervene with Russian authorities "in the strongest terms and demand an official explanation."

Moscow said it denied entry to Polish senate Speaker Bogdan Borusewicz and Latvian European lawmaker Sandra Kalniete because they were on a list of officials barred from visiting the country in retaliation for the EU sanctions against Russia.

Many commentators said that like other key opposition leaders, Nemtsov was constantly shadowed by police, so it would be hard to imagine 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.

A Ukrainian model who was with opposition Nemtsov when he was gunned down Friday was back in her country Tuesday. Anna Duritskaya has said she didn't see the gunman who killed Nemstov.

Nemtsov's killing was the biggest political assassination in Russia since another Kremlin foe, journalist Anna Politkovskaya, was shot to death in the elevator of her Moscow apartment building on Putin's birthday in 2006. 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 counting on it 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 Ukrainian crisis.

Armed with flowers and flags, tens of thousands turned out Monday in a somber but defiant display of solidarity with Nemtsov. Some carried banners that read "I am not afraid." But CBS News correspondent Clarissa Ward reports, in reality, many democracy activists in Russia acknowledge privately that they are now afraid.

Opposition leader Vladimir Ryzhkov was close friends with Nemtsov for more than 20 years. He said Monday that he can't remember the last time the atmosphere in Russia was as aggressive and negative as it is at present.

"This atmosphere was created by Kremlin propaganda for the last year," Ryzhkov told Ward. "I blame state and state propaganda for creating such kind of pro-violence, pro-terror and public atmosphere in country."

No matter who pulled the trigger, Ryzhkov said, the message was clear: Those who speak out against Putin can be targeted.

"I think that nobody knows, not only what future for opposition we have, no one can say what future for Russia we will have," Ryzhkov told CBS News.

Nemtsov had reportedly received death threats in the past and his friends claim that he was planning to publish further evidence of Russia's military involvement in Ukraine -- involvement that Russia still strenuously denies.

But former CBS News Moscow bureau chief Beth Knobel said Tuesday morning that Nemtsov's killing was a "big political problem now for Vladimir Putin."

She said that while opposition members are all aware of the risks they take, the brazen murder of such a prominent figure practically on the doorstep of the Kremlin has prompted an outcry from Russia's normally quite middle class.

Pointing to the huge crowd which gathered in the streets Monday, Knobel said a lot of those middle class Russians, "are saying 'you know, I've never been to a rally before... but what's happening to this country is a tragedy.'"

"This murder is bringing the opposition together," said Knobel.