Long gone are the fierce horsemen that protected the borders of czarist Russia, but Cossacks are still a part of Russian politics. Communities claiming Cossack heritage have reasserted themselves as ultraconservative factions loyal to President Vladimir Putin. (The Washington Post)

In an attack reflecting rising political violence in Russia, pro-Kremlin activists on Tuesday beat members of an opposition advocacy group led by a prominent whistleblower, injuring six people and leaving one hospitalized.

Aa group of Cossacks — wearing their signature fleece hats and yelling “Get off our land!” — punched and kicked activists from the Fund to Fight Corruption. The activists were returning from a team-building hike when they were attacked at the airport in the southern Russian city of Anapa, a spokeswoman for the group, Kira Yarmysh, said by telephone.

Among those assaulted was Alexei Navalny, an anti-corruption campaigner who has been the target of numerous criminal investigations and whose brother is serving a jail sentence on fraud charges.

The Cossacks, once-fierce horsemen who guarded the borders of czarist Russia, have now reasserted themselves as ultraconservative factions loyal to President Vladimir Putin.

A video frame from Anapa Today shows pro-Kremlin activists throwing milk at Russian opposition leader Alexei Navally, center right, at an airport in southern Russia on May 17. (Dmitry Slaboda/AP)

Aside from fielding units to fight alongside pro-Kremlin separatists in southeast Ukraine in 2014, they have increasingly focused their ire on the political opposition in Russia.

The Cossacks claimed Tuesday that Navalny’s supporters began the fight, pointing to a video that showed a young man in a red shirt and a backpack throwing an elbow during a standoff, sparking the brawl. The confrontation began after the Cossacks threw milk at Navalny’s supporters.

“Nobody planned an attack. They wanted to throw milk at them, which they did, and then tell them off to their face,” Dmitry Slaboda, a Cossack, told the radio station Govorit Moskva. “But, due to that strike, the fight began.”

Attacks against Russian opposition figures are becoming commonplace ahead of parliamentary elections in September. The Kremlin has warned against any repeat of the protests that erupted after the disputed elections in 2011.

However, violence and other forms of harassment against opposition leaders have only grown worse, cresting with the killing of former deputy prime minister Boris Nemtsov last year.

In recent months, members of Russia’s opposition have been subjected to low-level public attacks, often filmed and then uploaded to social-networking sites such as YouTube.

In February, several men approached opposition politician Mikhail Kasyanov at a restaurant and threw a cake at him. Kasyanov was also the target of a sex tape aired on state television last month that has split the opposition.

Late last month, Navalny was attacked in downtown Moscow with an “acrid, dark blue chemical liquid” on the same day that students and members of a civil rights society were targeted in a similar attack.

On Friday, a bus carrying Navalny and his colleagues was briefly halted for an inspection on the border of the southern region of Krasnodar as part of an anti-terrorism operation. Navalny wrote on Twitter that the group was being checked for connections to the Islamic State militant group.