MOSCOW (Reuters) - Thousands marched through central Moscow on Sunday to commemorate murdered Russian opposition leader Boris Nemtsov, calling for President Vladimir Putin to be ousted just three weeks before a presidential election.

Nemtsov, one of Putin’s most vocal critics, was shot dead on Feb. 27, 2015 as he walked across a bridge near the Kremlin. Aged 55, he had been working on a report examining Russia’s role in the conflict in Ukraine.

His killing sent a chill through opposition circles, and initiated annual marches in Moscow that have united different opposition parties and those discontented with the authorities.

Some blame Putin for Nemtsov’s death but he has never responded to such accusations. In 2015, he said the murder had a “provocative nature” and later that he was closely watching the investigation process.

“Nemtsov’s murder is a political act of terror. All the responsibility for the murder is on Putin,” presidential candidate for the Yabloko liberal party, Grigory Yavlinsky told Dozhd TV channel in a live broadcast from the march.

According to White Counter, an NGO that counts participants at rallies using metal detector frames, around 7,600 people took part in the Moscow march for which organisers had been granted permission by the city authorities.

Several hundred people also rallied in a central square in St. Petersburg, chanting “Russia will be free”, a Dozhd TV broadcast showed.

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In mid-2017 a Russian court sentenced Zaur Dadayev, a former soldier in Chechnya, to 20 years in jail for killing Nemtsov and handed terms of between 11 and 19 years to four other men convicted of being his accomplices.

But Nemtsov’s supporters said Dadayev and the others were only low-level operatives. The case remained unsolved, they said, because those who had ordered, financed and organised the hit had not been caught.

“We must make sure that those who ordered and organised the murder are detected,” Russian opposition politician Mikhail Kasyanov said in the Dozhd channel live stream from the march.

Ksenia Sobchak, another contender for the March election where Putin is widely expected to win, also took part in the rally, standing in the front line of a column that slowly moved through a 2.2 kilometre (1.4 miles) long route past metal fences guarded by police.

Opposition leader Alexei Navalny, who has been barred from running in the election, was also present, according to a Reuters cameraman.

People, marching at minus 14 degrees Celsius (7 degrees Fahrenheit), carried Russian flags and portraits of Nemtsov, occasionally chanting “Russia without Putin”, a usual slogan that can be heard at opposition rallies.

Carrying a tablet saying “I’m not afraid” and a bouquet of red roses, one participant, Andrei Biryukov from Moscow, told Reuters why he had come to commemorate Nemtsov.

“If our country was ruled by people like Nemtsov, we would live in a normal European country with free and independent people. That’s why I’m here,” said Biryukov.