MOSCOW (AFP) - Thousands of Russians marched through Moscow and Saint Petersburg on Saturday (Feb 27) in memory of opposition politician Boris Nemtsov, who was gunned down near the Kremlin a year ago in the highest-profile assassination of President Vladimir Putin's rule.

On a bright sunny afternoon opposition supporters thronged the streets in the Russian capital amid heightened police security as a helicopter hovered overhead.

Some marchers carried Russian flags, placards, flowers and Mr Nemtsov's portraits. Others chanted: "Russia will be free" and "Russia without Putin".

Some 20,000 joined the march, including Mr Nemtsov's allies - top opposition leader Alexei Navalny and former prime minister Mikhail Kasyanov, according to AFP journalists.

Moscow police, which are often accused of downplaying the popularity of opposition events, said 7,500 showed up.

Many protesters said the situation in Russia had got worse since the opposition politician's murder.

"Aggression and xenophobia have gone through the roof," Ms Anastasia Osipova told AFP.

"Over the past year things have become so much worse, both when it comes to the economy and freedom of speech," said the 20-year-old, clutching an EU flag.

"The authorities, this regime killed Nemtsov," said Mr Yevgeny Mishchenko, 41.

"The economic situation is worsening. Support for the authorities is crumbling. This will all end in a civil war, like a hundred years ago."

Russians also formed a huge line to lay flowers at the Great Moskvoretsky bridge where Mr Nemtsov, a jovial 55-year-old with a mop of black curly hair, was killed.

US ambassador John Tefft was among those who came to pay their respects earlier on Saturday, laying a wreath with a ribbon saying "From the American people".

Some said they would come to the makeshift shrine shortly before midnight, the time the politician was gunned down.

In Mr Putin's hometown of Saint Petersburg, some 4,000 people turned out to honour Mr Nemtsov.

"Putin is Russia's nightmare", one placard read, while some chanted "Putin get out".

"The authorities should know there are opponents," Ms Varvara Mikhailova, 24, said in the former imperial capital. "If we protest, something will change."

Russia's annexation of Crimea, fighting in Ukraine and Moscow's confrontation with the West have left the country deeply polarised.

Most of the population - who critics say have been under the spell of pro-Kremlin propaganda - support Mr Putin despite mounting economic troubles, while a minority says Russia is hurtling towards catastrophe.

Smaller commemorative events took place across Russia, while in London, Mr Nemtsov's eldest daughter Zhanna paid tribute to her father with the launch of a foundation in his name.

Backed by former tycoon Mikhail Khodorkovsky, an opposition leader who spent a decade in prison, the Boris Nemtsov Foundation for Freedom aims to continue Mr Nemtsov's mission of bringing democracy to Russia.

Britain's Europe Minister David Lidington urged Moscow "to ensure that those responsible for this appalling crime are brought to justice".

On the eve of the anniversary, lawmaker Dmitry Gudkov, one of the few independent voices in Parliament, suggested that deputies observe a moment of silence in Mr Nemtsov's memory, but most of his colleagues refused.

Mr Nemtsov, a former deputy prime minister in the government of Boris Yeltsin, was gunned down on Feb 27, 2015, while walking across a bridge a short distance from the Kremlin and Saint Basil's Cathedral with his Ukrainian model girlfriend.

Mr Putin, whose rule has seen the steady suppression of independent media and opposition parties, promised an all-out effort to catch the killers.

"Who dared?" a furious Mr Putin asked his aides after Mr Nemtsov was hit in the back by four fatal shots, the opposition Novaya Gazeta said.

Within weeks five men - all Chechens from Russia's restive North Caucasus - were arrested and charged with murder.

The detainees, including Zaur Dadayev, a member of a Chechen Interior Ministry battalion accused of being the gunman, are now awaiting trial for what investigators say was a carefully planned contract killing.

Mr Nemtsov's family and allies insist the authorities have failed to bring the masterminds to justice and point the finger of blame at Chechnya's Moscow-backed strongman Ramzan Kadyrov - and the Kremlin itself.

This month, men who were apparently from the North Caucasus threw a cake at opposition activist Kasyanov and shouted threats at him.

The Kremlin downplayed the attack, saying it should not be linked to Mr Kadyrov.

Two weeks later, unidentified attackers threw a cake at Mr Navalny.

Both attacks took place after the Chechen strongman posted an Instagram image of Mr Kasyanov in the crosshairs of a sniper's rifle and called the opposition "enemies of the people".

"They are trying to make the harassment of the opposition look like a farce but this does not mean that directors of the cheap comedies would refuse more brutal genres," said liberal daily Vedomosti.