MOSCOW (AFP) - Several thousand Russians and Western diplomats paid tribute at a Moscow bridge on Saturday (Feb 27) where opposition politician Boris Nemtsov was shot dead six years ago, as the United States said it was "deeply troubled" by the Kremlin's growing intolerance of free speech.

Mr Nemtsov was one of President Vladimir Putin's loudest critics until he was killed on a bridge near the Kremlin on Feb 27, 2015.

The sixth anniversary of his murder comes just after Putin's current most prominent critic Alexei Navalny was sent to a penal colony to serve a nearly three-year term he denounces as politically motivated.

The move has heightened already tense relations with the West, with the European Union moving to sanction Moscow.

From Saturday morning, a steady stream of Russians and Western diplomats took turns laying flowers at a makeshift memorial at the spot where Mr Nemtsov was felled with four bullets.

"We come together in this place on this day every year to show the authorities that we have not forgotten and will not forget," former prime minister and opposition politician Mikhail Kasyanov told journalists at the rally. "I am sure that what Boris fought for - freedom for Russians, their well-being and a dignified life - will soon come about," he said.

The anniversary is usually marked by a march through central Moscow by the opposition.

But they chose to rally this year at the makeshift memorial, which is regularly dismantled by the authorities, as restrictions on mass gatherings are still in place due to the coronavirus pandemic.

The White Counter monitor, which tallies attendance at rallies, said 7,800 people had gathered by 5.30 pm.

"He spoke the truth and for this in our country you get killed," Irina Drozdova, a 44-year-old lawyer, told AFP before laying flowers at the memorial.

Mr Navalny's wife Yulia Navalnaya was there as well and brought a bouquet of red carnations.

Also laying flowers Saturday were the US, British and EU ambassadors to Russia among other Western representatives.

'Inspiration to many'

US Embassy spokeswoman Rebecca Ross said US Ambassador John Sullivan had paid tribute to a man who was "dedicated to pursuing a better future for his country".

"He remains an inspiration to many who strive for justice, transparency, freedom," she tweeted.

US Secretary of State Antony Blinken released his own statement, saying he was "deeply troubled" by Russia's growing intolerance of free speech.

He called Mr Nemtsov a man who "dedicated his life to building a free and democratic Russia."

In 2017, a court found a former security force officer from Chechnya guilty of Mr Nemtsov's murder and sentenced him to 20 years in prison.

Four other men were found guilty of involvement in the killing.

But the slain opposition politician's family and supporters insist the authorities have failed to bring the masterminds to justice.

More than five years after Mr Nemtsov's death, Mr Navalny in August suffered a near fatal poisoning attack with the Soviet-era nerve agent Novichok that he blames on Putin.

The Kremlin has repeatedly denied the claim.

After spending months in Germany recovering from the poisoning, the 44-year-old opposition politician was arrested upon his return to Russia last month, sparking mass protests that resulted in some 11,500 demonstrators being detained.

The European Union has agreed to sanction four Russian officials over the crackdown on Mr Navalny and his supporters, and Blinken on Saturday referred to the attack on him.

"Those who would speak out in defence of their freedoms and democracy in Russia continue to be targeted for attack and assassination," he said. "The Russian people deserve better."