Supporters of slain Russian opposition leader Boris Nemtsov paid tribute to him on Saturday, gathering in Moscow a year after he was shot to death on a bridge near the Kremlin.

Nemtsov, a former deputy prime minister and fierce critic of President Vladimir Putin, was slain on Feb. 27, 2015 while walking with a companion.

Rallies in memory of Nemtsov were held in dozens of Russian cities, including in St. Petersburg, where a few  thousand people turned out, but most were small.

Nemtsov memorial

A line of flowers can be seen on the bridge near the Kremlin, where one year ago, opposition leader Boris Nemtsov was assassinated. (Susan Ormiston/CBC)

In Voronezh, a few dozen people took to the streets and unknown young men attacked the protesters with green dye and flour. In Nizhny Novgorod, the capital of the region where Nemtsov served as governor in the 1990s, several hundred people participated, including the mayor.

City authorities in Moscow denied march organizers permission to hold a procession to the bridge, but gave permission for another route in central Moscow on Saturday.

A line of flowers continued to be built Saturday on the Bolshoy Moskvoretsky bridge in honour of Nemtsov.

The U.S. ambassador to Russia, John Tefft, was among those who paid tribute. He laid a wreath at the site where Nemtsov, 55, was killed.

Boris Nemtsov first-year anniversary memorial

People brought flowers and candles to the bridge where opposition leader Boris Nemtsov was gunned down a year ago, paying their respects before attending a march in his honour in central Moscow. (Corinne Seminoff/CBC)

"I am here this morning with my deputy and my staff representing the president and the people of the United States to honour the memory of a man who we knew as a government official, as a politician and, for many Americans, as a friend. We are here today to honour his memory, the values for which he stood and to express our hope for the future that some of the dreams that Boris Nemtsov had will come true in Russia." Tefft said.

Several Chechen men were arrested shortly after the murder, including five people who are due to go on trial later this year.

The suspected triggerman served as an officer in the security forces of the Moscow-backed Chechen leader Ramzan Kadyrov.

The official probe has failed to identify those who ordered the killing, and Russian opposition activists have criticized the Kremlin for it.

Anton Nemtsov

Nemtsov 's son, Anton, observes a moment of silence at his father's memorial. (Susan Ormiston/CBC)

"But the thing here is people believe, many in the opposition believe, that the person who ordered the killing has not been identified and will not be identified," CBC journalist Susan Ormiston reported from Moscow. "They believe that the orders came from much higher up."

The Russian government has turned down numerous requests to put up a monument or plaque to commemorate Nemtsov. Opposition activists have also been thwarted in their attempts to get the name of the bridge changed to Nemtsov Bridge.

Opposition leader Mikhail Kasyanov, a close friend of Nemtsov, brought flowers to the bridge on Saturday. "They were afraid of you then, they're afraid of you now," he said.

Mikhail Kasyanov

Opposition leader Mikhail Kasyanov laid a large bouquet of red roses at the memorial to Boris Nemtsov on a bridge in the shadow of the Kremlin. 'They were afraid of you then, they're afraid of you now, he said. (Corinne Seminoff/CBC)

Last week, Kasyanov, 58, told Reuters he himself fears for his life after the killing, leaving him as the most high-profile Kremlin opponent inside the country.

Kasyanov was shown in the cross-hairs of a sniper's rifle in a video posted on the internet earlier this month by the pro-Russian boss of Chechnya, a Putin ally known for scathing condemnations of the Kremlin's enemies.

Days later, two men accosted Kasyanov in a Moscow restaurant, rubbed a cake in his face and threatened him.

Despite the intimidation, supporters of the slain opposition leader still gathered for the approved march in central Moscow.

'Honest conscience of our country'

Olga Machelovo came to the bridge to leave two roses at the Nemtsov memorial. She's in her 80s and has been coming every month with two roses on her "miserly pension."

"There are people who can be replaced and there are people who can never be replaced and he is a person no one can replace," she said.

"He was the honest conscience of our country. To be killed, on a bridge right by the Kremlin, this tells you something."

"He got in the way of the government," Machelovo said. "They got rid of him for this."

Mikhail Kasyanov (left) and Boris Nemtsov

Kasyanov, left, and Nemtsov in 2014. Nemtsov was killed Feb. 27, 2015, and some members of the opposition believe it was on the orders of pro-Putin Chechen leader Ramzan Kadyrov. (PARNAS)

A woman who had joined the march said an impressive number of people came out and that "Boris Nemtsov is uniting people in death," that "people should not stay silent."

"I am against our corrupt government," said another marcher named Kate. Asked if she was encouraged by the turnout she said, "I am not. We've seen things like this before and nothing changes, what can we do?"

The march ended without any serious incident. An official estimate put the crowd at 5,000 people but protests in Russia are often underestimated by authorities. The Associated Press reported about 30,000 people marched across Moscow on Saturday, chanting, "Russia will be free" and some holding signs saying, "I am not afraid."

Boris Nemtsov memorial Moscow

Pensioner Olga teared up as she told CBC that she has come to the Nemtsov memorial every month, leaving flowers, on a miserly pension. 'There are not enough people like him,' she said. (Susan Ormiston/CBC)

With files from The Associated Press and Reuters