As millions of Russians vote in parliamentary elections this weekend, one of the stand-out features of the campaign has been the presence of the opposition.

In past votes, most opposition candidates have been blocked or excluded. But in this election, taking place on Sunday, hundreds of Kremlin critics have been allowed to run for office.

Some have even been given air time on Kremlin-controled state television, which is normally free of any opposition voices.

"[The authorities] think they should create some kind of picture that elections are free and fair in accordance with international standards," said Mikhail Kasyanov, leader of the PARNAS opposition party.

But that picture is not accurate, say critics.

Opposition figures have complained of threats and harassment.

Sinister videos

Kasyanov, a former prime minister turned vocal critic of Russian President Vladimir Putin, has been targeted repeatedly.

In one incident, caught on camera, he was attacked in a restaurant by two men wielding a cream pie.

In what critics say was another bid to humiliate and discredit the former premier, a secretly-filmed sex tape was also circulated showing the opposition leader in a compromising embrace with a married associate.

And it gets more sinister: One video posted online showed Kasyanov and another opposition figure in the cross-hairs of a sniper's rifle.

The head of the Chechen republic, Ramzan Kadyrov -- a key Putin ally -- said he posted the clip, insisting it was a joke.

But in a country where critics of the Kremlin are routinely murdered, it is no laughing matter.

Opposition figure killed

In February 2015, another leading Russian opposition figure, Boris Nemtsov, was gunned down on a bridge near the Kremlin, as he walked home from a restaurant.

Kasyanov and Nemtsov were close associates.

In May last year, another opposition figure, Vladimir Kara-Murza, fell violently ill and was initially diagnosed with poisoning.

After being released from hospital, Kara-Murza said he believed he had been targeted by Kremlin supporters.

"These days in my country, unfortunately, everyone should be scared about the behavior of the authorities or other people," Kasyanov told CNN at his party headquarters.

"Me too. I am a normal person, that is why I am also scared and I can expect something to happen to me and my family."

"But I have to continue this mission, this job, which we are already committed to do," he added.

Rare television debate

Despite the ratcheting up of pressure on activists, Kasyanov says the current official willingness to allow Kremlin critics to stand for election and be given air time is an opportunity for the political opposition.

He was recently invited to take part in a rare televised debate on state television, where most Russians get their news.

In a stunt during the live proceedings, a pro-Kremlin candidate planted a US flag on Kasyanov's lectern, shouting that it was a reminder of which country's interests Russia's opposition defended.

But Kasyanov says the fact he was on state television at all is what is important.

Up until that debate, he said, he had not been invited to appear on national television for nearly a decade.

"It's angering some people, but others are starting to wake up," said Kasyanov says. "They wake up and say it is possible -- even in a situation where everything seems to be under total control of Putin -- to appear on the first channel.

"And they started thinking that something could be changed in the country."

(CNN)