A video posted on Russian social media purports to show Alexei Navalny, an opposition politician, and fellow anti-corruption activists being attacked, thrown to the ground and kicked by a group of men at an airport in south Russia.

Tuesday's attack showed Cossacks wearing traditional fur hats, some in military-style fatigues and black boots, throwing milk at the group outside Anapa airport before assaulting them.

"The Cossacks and the police carried out the attacks but of course it was the authorities who organised them," Navalny, 39, said on social media.

He was travelling with his wife and young children when the assault occurred.

Dmitry Peskov, a spokesperson for the Kremlin, said: "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?"

Police response

According to Navalny, the attackers included about 30 Cossacks.

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

Navalny said the police had followed his group during their four-day visit in the area.

The police told the Interfax news agency they were investigating the attack but had not made any arrests.

Al Jazeera's Rory Challands, reporting from Moscow, said Navalny and his spokesperson have been saying that they believe this was a premeditated attack.

"They said they've been hassled on their trip in southern Russia and this was sort of a continuation of a campaign of intimidation that they have been under," he said.

Navalny, who has taken on leading officials in President Vladimir Putin's government, is a prominent leader of a 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.

Cossack account

For his part, 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.

Things turned violent after one of them elbowed an elderly Cossack to the ground, he said.

"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 on the ground after being kicked in the head.

Navalny said the man had to be hospitalised. Five others, including Navalny himself, also suffered injuries.

Cossack paramilitaries, who style themselves as conservative patriots, helped Russia to annex Crimea from Ukraine in 2014.

Cossack militia members attacked the Pussy Riot punk group with whips and tear gas at the 2014 Winter Olympics in Sochi as they tried to perform a song mocking Putin.

State media and pro-Kremlin politicians have repeatedly portrayed Russia's 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.