Russian opposition leader Alexei Navalny and fellow anti-corruption activists were attacked, thrown to the ground and kicked by a group of men at an airport in southern Russia, according to video posted on social media.
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 1500 kilometres south of Moscow, before setting upon them on Tuesday (local time).
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. Navalny said the man later had to be hospitalised. 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 tear gas at the Winter Olympics host city of Sochi in 2014 as they tried to perform a song mocking Putin.
Navalny, 39, who has accused top government officials from President Vladimir Putin 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.
Navalny 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 organised 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.
The police told the Interfax news agency they were investigating the attack.
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?"