Russian police detained around 400 people on Wednesday, including opposition politician Alexei Navalny, at a protest in Moscow calling for punishment for police officers involved in the alleged framing of a journalist, monitors said.

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 flatly 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 the streets of central Moscow, chanting "Russia will be free" and "Russia without Putin" 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 front page headline carried by Russia's three leading daily newspapers on Monday.

Many of the marchers and those detained 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 pose a risk to 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. Organisers of Wednesday's event had demanded that Moscow city officials negotiate those terms with them during a live broadcast, something they said officials refused to do.

OVD-Info, a monitoring group, said police had detained around 100 people, some of them by force.

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

Sergei Udaltsov, a leftwing political activist, said the Golunov case was an important moment. "If we do not show solidarity with each other, the system will break all of us one by one," he said.

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