(Reuters) - Thousands of stunned Russians laid flowers and lit candles on Saturday on the bridge where opposition politician Boris Nemtsov was shot dead near the Kremlin, a murder that showed the risks of speaking out against President Vladimir Putin.
Nemtsov, 55, was shot four times in the back by killers in a white car late on Friday as he walked across the bridge over the Moskva River in central Moscow with a Ukrainian woman, who was unhurt, police said.
Police sealed off the blood-stained bridge close to the red walls of the Kremlin and Red Square for two hours overnight, then hosed it down as people came to pay tribute to one of Putin's biggest opponents over Russia's role in Ukraine.
Russia's Investigative Committee, which answers to Putin, said it was following several lines of inquiry, including that the opposition may have committed the crime to rally support for a march against Kremlin policies on the economy and Ukraine.
Flowers were piled at least a meter (three feet) high, about two meters deep and two meters wide. A piece of white paper saying "We are all Nemtsov" stood among the flowers.
"People are afraid to support our movement. Opposition activists receive threats every day and Boris was no exception. But they won't stop us," said opposition activist Mark Galperin.
No government or Kremlin official was seen paying tribute but many opposition figures did so, with some warning that the pro-war mood and anti-Western hysteria whipped up by Putin over Ukraine was leading Russia into a dark future.
"In our country there is demand for anger. In our country there is demand for hatred. In our country there is demand for aggression," said Anatoly Chubais, late President Boris Yeltsin's chief of staff and a liberal economic reformer.
Referring to a pro-Putin march last week, he said: "If just a few days ago here in this city people were marching with a poster "Kill off the fifth column" and then today Boris Nemtsov is killed - let's just pause and think what will happen tomorrow. We all must stop."
A former deputy prime minister who had feared Putin wanted him dead, Nemtsov was the most prominent opposition figure killed in Putin's 15-year rule. At one time he had been widely seen as the man most likely to succeed Yeltsin as president.
His gangland-style murder was reminiscent of the chaotic 1990s after the Communist Soviet Union collapsed and raised further questions about the opposition's ability to mount any challenge to Putin in such a dangerous environment.
The Kremlin deflected accusations that it was to blame and Putin put the investigation under presidential control, denouncing what he called a "provocation" before an opposition protest that had been planned on Sunday.
In a telegram to Nemtsov's mother, he promised the killers would be found and punished.
But the killing focused attention on the tough treatment of opponents in Putin's third term, during which several leading critics have been jailed or have fled Russia following mass rallies against the former KGB spy three years ago.
"I would say this is not only a blow to the opposition, it is a blow to all Russian society. It is a blow to Russia. If political views are punished this way, then this country simply has no future," said Sergei Mitrokhin, an opposition leader.
FEW POLITICAL MURDERS SOLVED
Leading international condemnation of the murder, U.S. President Barack Obama called for a prompt, impartial and transparent investigation to ensure those responsible were brought to justice for the "vicious killing".
Political murders often go unsolved in Russia. Police said they were investigating whether the murder was aimed at destabilizing the political situation in Russia or was committed by radical Islamists against Nemtsov, a Jew.
A car suspected of being used by the killers, and identified as coming from the mainly Muslim Ingushetia region, was found abandoned in central Moscow. Some Russian news outlets said surveillance footage showed two men leaving it.
Former Soviet President Mikhail Gorbachev cautioned against jumping to conclusions but some opposition figures blamed Putin directly. Others said Russian society was in decline, describing an environment where Putin demands total loyalty and supporters go to great lengths to do what they think may please him.
"In Putin's atmosphere of hatred and violence, abroad and in Russia, bloodshed is the prerequisite to show loyalty, that you are on the team," another opposition leader, former world chess champion Garry Kasparov, said on Twitter.
"If Putin gave (the) order to murder Boris Nemtsov is not the point. It is Putin's dictatorship. His 24/7 propaganda about enemies of the state."
Nemtsov, who had been out walking on Great Moskvoretsky Bridge after a meal in a restaurant by Red Square, had said in a recent interview the president might want him dead because of his opposition to the conflict in Ukraine.
Ukrainian President Petro Poroshenko said Nemtsov had told him a couple of weeks ago that he planned to disclose evidence of Russia's involvement in Ukraine's separatist conflict.
"Someone was very afraid of this ... They killed him," Poroshenko said in televised comments shown in Ukraine.
Kiev, the West and some Russians accuse Moscow of sending troops to support separatist rebels who have risen up in east Ukraine, an accusation Russia has denied.
The organizers of Sunday's planned protest against the war decided to cancel it. Instead, Moscow city authorities agreed they could hold a march for up to 50,000 people to remember Nemtsov.
Nemtsov's criticism of Putin won him support among Moscow's intellectuals and the nascent middle class but he had little support outside the big cities.
Nemtsov was a fighter against corruption. In other reports, he condemned overspending on the 2014 Sochi Winter Olympics by the Russian authorities and listed the many state buildings, helicopters and planes that Putin has at his disposal.
He was also one of the leaders of the rallies in the winter of 2011-12 that became the biggest protests against Putin since he first rose to power in 2000.
Nemtsov briefly served as a deputy prime minister under Yeltsin in the late 1990s, after winning a reputation as a leading liberal economic reformer as governor of the Nizhny Novgorod region.
The opposition has failed to dent Putin's popularity even though many people feel the pain of Western economic sanctions over Ukraine, low oil prices and poor economic management.
Opposition blogger Alexei Navalny is serving a 15-day jail term. Kasparov is based in the United States and former oil tycoon Mikhail Khodorkovsky, freed in late 2013 after a decade in jail, lives in Switzerland.
Some opponents say they fear for their lives. Anna Politkovskaya, an investigative journalist, was shot dead outside her Moscow apartment on Putin's birthday in 2006. The person who ordered the killing has never been identified.
(Additional reporting by Katya Golubkova, Vladimir Soldatkin, Denis Dyomkin, Polina Devitt and Thomas Grove, and by Roberta Rampton in Washington; Writing by Timothy Heritage; Editing by Elizabeth Piper and)