Russian opposition leader Alexei Navalny and supporters attacked

Footage showed the assailants, described by Navalny as Cossacks and wearing traditional fur hats, some in military-style fatigues and black boots, showering the group with milk outside the terminal of Anapa airport, about 1,500 kilometers (900 miles) south of Moscow, before setting upon them.

Dmitry Slaboda, one of the Cossacks, said the original plan had been just to throw milk at Navalny and his supporters and to hurl insults at them, but that things had turned violent after one of them had elbowed an old cossack to the ground.

"The fight broke out because of that blow," Slaboda told the Govorit Moskva radio station. "We just wanted to show them that there is no room here for Navalny who lives on American money."

One of the activists could be seen lying prone on the ground after being kicked in the head. Navalny said the man later had to be hospitalized. Five others, including Navalny himself, also suffered injuries.

Cossack paramilitaries, who style themselves as conservative patriots, helped Russia annex Crimea from Ukraine in 2014. Cossack militia members also attacked the Pussy Riot punk group with whips and teargas at the Winter Olympics host city of Sochi in 2014 as they tried to perform a song mocking Vladimir Putin.

Navalny, 39, who has accused top government officials from the president downwards of graft, is one of the most high profile leaders of the country's small liberal opposition and the founder of Russia's Anti-corruption Foundation.

Russia is due to hold parliamentary elections in September and Navalny and his group were returning from a team-building weekend in the area when they were assaulted.

State media and pro-Kremlin politicians have repeatedly cast the liberal opposition, which has only one sympathetic deputy in the 450-seat lower house of parliament, as a foreign-funded fifth column intent on sabotaging the country.

Navalny, who was traveling with his wife and young children, said the attack had been carried out by about 30 Cossacks and that they had punched women as well.

He accused the police of doing nothing to stop the violence and of having helped plan the attack.

"The 'Cossacks' and the police carried out the attacks, but of course it was the authorities who organized them," Navalny said on social media, adding that the police had followed his group during their four-day visit, even detaining them once as part of what the authorities termed an anti-terrorism operation.

Navalny and his supporters filed a report about the attack to the police, which also alleged police inaction.

The police told the Interfax news agency they were investigating the attack, but had so far made no arrests.

Dmitry Peskov, a spokesman for the Kremlin, when asked about the episode, told reporters: "I would not want to interpret it based on just one source...Was it really an attack? And if it was, who carried it out, who turned to the police later?"

Vladimir Ashurkov, a Navalny ally based in London, called the incident an escalation of government-backed pressure on civil and political activists.

"This outrageous act is reminiscent of the activities of Germany’s HitlerJugend or China’s Red Guards, exercising lawless harassment with (the) tacit approval of the authorities," he said in a statement. "It represents another low."

(Additional reporting by Dmitry Solovyov; Editing by Ralph Boulton)