Police abruptly dropped drug charges a day earlier against investigative journalist Ivan Golunov, a rare U-turn by the authorities in the face of anger from his supporters who said he was targeted over his reporting.

Golunov, 36, known for exposing corruption among Moscow city officials, was detained by police last Thursday and accused of dealing drugs, an allegation he denied.

The crude way supporters said Golunov was set up and detained triggered an unusual show of media unity and an uncharacteristically swift reversal from authorities nervous about social unrest at a time when President Vladimir Putin already faces disquiet over living standards.

The authorities had hoped freeing Golunov and promising punishment for those who allegedly framed him would appease his supporters, but they decided to go ahead with a protest on Wednesday, a public holiday in Russia, regardless.

Reuters witnesses said well over 1,000 people marched through central Moscow, chanting "Russia will be free", "Russia without Putin" and "Down with the Tsar" as police warned them not to break the law and blocked access to certain streets.

Some of the protesters wore white T-shirts saying "I am/We are Ivan Golunov," the same slogan as a front page headline carried by Russia's three leading daily newspapers on Monday.

In an unexpected twist, a young woman wearing such a T-shirt managed to get a front-row seat at a ceremony in the Kremlin at which President Vladimir Putin was handing out state awards, official footage of the event showed. The ceremony was taking place at the same time as the protest in the city.

OVD-Info, a monitoring group, said police had detained over 400 people. It said police had started to release some of them without charge while drawing up charges against others.

Police said earlier they had detained over 200 people.

Many of the marchers and those forcefully detained by riot police were prominent Russian journalists and activists.

"We came to show the authorities that we have consolidated, that we are united," said Vsevolod, 24. "We demand that hundreds of thousands of (criminal) cases where people are sitting in prison unfairly now be reviewed."

The authorities had warned protesters that their demonstration would be illegal and could threaten public safety.

Under Russian law, the time and place of protests involving more than one person needs to be agreed with the authorities in advance. Organizers of Wednesday's event had demanded that Moscow city officials negotiate those terms with them live on air during a TV broadcast, a demand they said officials refused.

A Reuters witness saw at least three police officers bundle opposition politician Alexei Navalny into a truck.

Kira Yarmysh, his spokeswoman, said on Twitter that Navalny was accused of breaking Russia's protest laws, something he has been repeatedly found guilty of, and faced up to 30 days in jail.

Navalny, who said police had accused him of organizing the march, said on Twitter he had been glad to march "among honest people".

One protester, Ivan, 28, explained why marchers had defied the police. "The freeing of Golunov was not a victory. It was a tactical move by the authorities to prevent the disorder from breaking out today. But we came here anyway."

REUTERS