Nemtsov, a critic of President Putin, was gunned down on February 27, 2015, close to the Kremlin. Thousands of Russians marched on Sunday to commemorate his life. The US is calling for those who ordered the crime to be held to account.
Four years ago, on February 27, 2015, Boris Nemtsov was gunned down shortly before midnight, walking across a bridge a short distance from the Kremlin in the heart of Moscow. He was 45 years old.
Nemtsov rose to prominence as a liberal politician during the era of Boris Yeltsin, but he is remembered today as a vocal critic of Russia's President Vladimir Putin.
Several thousand Russians gathered in Moscow on Sunday in commemoration of his death.
Ordinary Muscovites and prominent politicians, including opposition leader Alexei Navalny, marched through the centre of the city.
Many carried Russian flags and portraits of Nemtsov. There were anti-Kremlin banners and anti-Putin chants.
Six thousand people took part, police said in a statement on the interior ministry website. Pro-opposition monitors estimated the turnout at 10,000.
A disappearing future
"It's about the future of the country that Putin is taking away from the people," opposition politician Ilya Yashin said.
He said a growing number of young people were taking part in opposition marches.
Ivan, a 55-year-old driver who refused to give his last name, said he joined the march because Nemtsov "was a clean person with democratic principles."
"He was killed because the authorities had had enough of him," he said.
Marches were held in other cities, including Saint Petersburg, where around 800 people gathered to mark the anniversary.
"I come to this event every year. Unfortunately nothing is changing in the country, and we cannot be silent," said 50-year-old Galina Apraksina.
"We need to show that there are people in the country that do not agree (with the Kremlin's course) – as long as we still have the possibility to do so," said 38-year-old Ivan Svishenko.
Kremlin critics say there has been a clampdown on civil society and dissent since Putin came to power almost 20 years ago.
In 2017, a court found a former security force officer from Chechnya guilty of murdering Nemtsov and sentenced him to 20 years in prison. Four other men were found guilty of involvement in the killing and also sentenced to lengthy prison terms.
US calls for accountability over Nemtsov's murder
The US on Tuesday call on Putin's government to "identify publicly those who organized and ordered the crime, and hold them accountable."
In a statement issued on the eve of the anniversary of Nemtsov's fatal shooting, the State Department said Nemtsov "remains an inspiration to all those in Russia and around the world who strive for justice, democracy, and a government accountable to the people."
Relatives and associates of the Kremlin critic believe his killing was ordered at a higher level and say justice will not be served until the person or people behind it are identified and prosecuted.
The State Department said the United States honors Nemtsov's memory "by renewing our call for the government of Russia to allow journalists, civil society activists, and political opposition members to exercise their universal human rights of freedom of expression, association, and peaceful assembly without fear of violence or other forms of reprisal."