Russian officials on Thursday faced growing outrage at home and abroad over the jailing of Kremlin critic Alexei Navalny and a crackdown on protesters, with the US threatening to take action over Moscow's "malign" behaviour.

EU foreign policy chief Josep Borrell is on a three-day trip to Russia -- the first by a top EU envoy since 2017 -- and he is under pressure to raise the issues during high level meetings on Friday.

Police have detained at least 10,000 people during recent nationwide protests in support of Navalny, the most prominent critic of President Vladimir Putin.

He was sentenced this week to nearly three years in prison -- his first lengthy jail term -- and is due in court again on Friday on charges of defaming a World War II veteran.

Over the past few weeks police detained thousands of protesters and dozens of journalists
Over the past few weeks police detained thousands of protesters and dozens of journalists AFP / Kirill KUDRYAVTSEV

Navalny urged his supporters to fight fear and free Russia from "a handful of thieves in power" on Thursday in a message from prison, his first detailed comments since the court ruling.

During the recent demonstrations police detained dozens of journalists, with jails packed full of demonstrators serving short sentences in Moscow and Saint Petersburg.

Leonid Volkov, a close aide to Navalny, called for fresh demonstrations for later in the year rather than next weekend, easing the standoff with the Kremlin.

"We will properly organise them and definitely hold another big one in spring and summer," he said in a YouTube live stream.

The jailing of Navalny, 44, and the crackdown sparked outrage among many Russians
The jailing of Navalny, 44, and the crackdown sparked outrage among many Russians AFP / Alexander NEMENOV

Prominent rights campaigners in Russia said they were concerned by the "unprecedented escalation of baseless violence", in a joint statement put out by leading rights group Memorial.

"Never in the history of modern Russia has there been such a number of beaten, detained and arrested people," they said.

Navalny's allies called on Russians to take to the streets after he was detained last month
Navalny's allies called on Russians to take to the streets after he was detained last month AFP / NATALIA KOLESNIKOVA

Top broadsheets added their voices to the chorus of condemnation.

"Over the past few weeks we've witnessed extremely harsh actions of members of law enforcement," Kommersant said.

"Beatings and mass detentions should not become the norm in our country."

Business newspaper RBC demanded that law enforcement explain the arrest of Sergei Smirnov -- chief editor of Mediazona, an online news publication often critical of the government -- and other journalists.

Borrell insists he will deliver "clear messages" to Moscow
Borrell insists he will deliver "clear messages" to Moscow POOL / JOHN THYS

Navalny's allies called on Russians to take to the streets after he was detained last month on arrival from Germany where he had been recovering after being poisoned.

Borrell is eager to sound out Russian Foreign Minister Sergei Lavrov on issues including enlisting Russia's help in reviving the Iran nuclear deal and tackling climate change.

But the jailing of Navalny and the police crackdown are expected to dominate the agenda when the two diplomats meet on Friday.

Borrell insisted he would deliver "clear messages" to Moscow, which has shrugged off Western calls to free Navalny, who has accused Putin of trying to kill him.

On Thursday, EU foreign policy spokesman Peter Stano called the talks "a delicate diplomatic balancing act".

The EU's ties with Russia have been in the doldrums since Moscow seized Crimea in 2014 and fuelled a war in Ukraine that claimed more than 13,000 lives.

On Thursday, Putin's spokesman Dmitry Peskov said Brussels and Moscow should be able to freely discuss "all existing differences".

Kremlin critics allege a concerted effort to intimidate dissenters and put pressure on Navalny's family and allies, some of whom have been placed under house arrest for two months.

Moscow's jails were bursting at the seams following the crackdown, and many protesters were sent to a detention centre for migrants outside the capital.

On Thursday, an AFP journalist saw around 100 people queueing outside the Sakharovo migrant centre, waiting to pass care packages to detainees.

"This is just an incredible disgrace," 27-year-old Denis Bondarenko said outside the centre, noting his cousin had been randomly snatched off the street.

Calls are growing in Europe for the EU to slap new sanctions on Moscow. An EU statement said foreign ministers would discuss "possible further action" at a meeting on February 22.

US President Joe Biden's top security adviser Jake Sullivan, meanwhile, said the new administration would take action against Moscow in due course.

"We will do that at a time and a manner of our choosing," he said.

Separately, US Secretary of State Antony Blinken voiced concern over Navalny in a call with Lavrov, the State Department said.