The assault on the 36-year-old father-of-two is one of a growing number of vicious attacks on opposition figures in the run-up to a presidential election in March which Putin, the incumbent, is widely expected to contest, Reuters said.

Most activists do not believe that Putin or the Kremlin have a hand in the attacks, which have included caustic liquid being thrown in a victim's eyes, a car being set alight, and, in one case, an activist being bashed over the head with an iron bar.

But critics say the way the authorities have handled the cases – it's rare for anyone to be arrested and a nationalist group which says its carries out such attacks openly boasts about its activities – shows that they are at best turning a blind eye, and at worst tacitly condoning the violence.

Read alsoMoscow Times: Five men found guilty in Nemtsov murder caseYulia Latynina, a journalist critical of the authorities, was forced to flee Russia this summer after having faeces thrown at her, her car torched, and noxious gas pumped into her home.

For her, Skripnichenko's death is part of a dangerous trend, and she has accused the authorities of losing control of the violent extremists responsible.

"When you splash antiseptic in people's faces, pour shit over people, beat up activists, puncture their tires, or burn something, sooner or later someone will be burned alive or die in a fight," Latynina said on her own radio show.

"And that's murder."

According to Skripnichenko's family, an unidentified man accosted him late on Aug. 15 as he stood by a makeshift memorial to slain opposition leader Boris Nemtsov on a Kremlin bridge, a rallying point for government critics.

"Don't you love Putin?" the man asked Skripnichenko.

"Is Putin a girl that I should love him?" the activist parried.

The man knocked Skripnichenko's cap to the ground, and, as his victim rose after picking it up, punched him hard in the face, breaking his nose. Skripnichenko hit his head on the pavement as he fell. His attacker then kicked him as he lay prone.

Mikhail Silich, an activist who was with Skripnichenko at the time of the attack, confirmed the family's account.

After the attack, Skripnichenko left hospital and told other activists he thought he would be OK. His family say he went to a different hospital a week later to get his nose reset, and began to feel unwell in the canteen. When he didn't come home that day, they phoned the next morning to be told he had died.

His family and activists say the authorities are not investigating his death properly.

Investigators have neither identified nor arrested Skripnichenko's attacker, have not opened a criminal case, and the dead man's family say they can't obtain his autopsy report.

Citing procedure, Moscow's Investigative Committee, which is handling the matter, told Skripnichenko's widow Olga that she would need to wait 40 days to get such a report.

The committee did not respond to a Reuters request for comment on progress in the case.

Despite the attack taking place in sight of the Kremlin's walls on the same spot where Nemtsov was gunned down two and a half years earlier, investigators say there is no CCTV footage. Ilya Novikov, the family lawyer, says that's hard to believe.

The cause of death is in question too. Investigators' preliminary conclusion was that he had died of heart disease.