Vladimir Putin is no friend of the Jews. So says William Browder, the American-born Jewish tycoon who describes himself as the leader's "number one enemy" in his book: Red Notice.

Browder, 51, is on a mission to bring down the Russian president, the feared leader who has helped transform this world-renowned financier into a leading human rights activist.

I have come to meet Browder, the founder of investment fund Hermitage Capital Management, who made his fortune as the largest foreign investor in Russia until 2005, at his central London offices.

Ever since he was deported from Moscow in 2005, he has set his sights on exposing corruption in Russia and Putin, whom he once supported, as a: "sociopath", "cold-blooded killer" and "criminal dictator who is not too different to Hitler, Mussolini or Gadaffi".

With undertones of a thriller, Browder tells his (almost unbelievable) story through his book. After his expulsion, the Kremlin served an unsuccessful red notice (similar to an international arrest warrant) to Interpol against Browder - indicting him for tax evasion in absentia. In turn, Browder hired a legal team to protect him. One of those Russian lawyers, Sergei Magnitsky, discovered that officials had stolen $230 million as part of an elaborate tax fraud.

As a result of the exposure, Magnitsky, a 37-year-old father of two, was imprisoned and tortured for 358 days and in 2009, brutally beaten to death.

The death still haunts Browder. His voice quietens and he looks away at any mention of Magnitsky; he feels responsible for his death and is now determined to get "justice".

Equally, he stiffens at any mention of Putin that may be construed as positive.

Some people, I say, would consider Putin to be a friend of the Jews - or, at least, not an enemy. After all, he donated one month of his salary towards the construction of Moscow's Jewish Museum and Tolerance Centre, the largest Jewish museum in the world. He appears to be "friendly" with leading communal figures, including Moshe Kantor, president of the European Jewish Congress and Russian Chabad Chief Rabbi Berel Lazar. Moreover, he did send a Rosh Hashanah message to the Jewish community - so surprisingly sweet it went viral.

In September, Putin even posed for a meet, greet and handshake with Israeli Prime Minister Benjamin Netanyahu, as the two leaders pledged to co-ordinate military actions over Syria.

"He has got a lot of allies in the far-right who are virulently antisemitic," Browder hits back. "Putin is pretty smart about who he should or should not alienate. He knows that, if he alienates Jewish people, he alienates a very powerful group of people around the world."

He continues: "Putin has sent over several thousand troops to Syria to effectively engage on behalf of Bashar al-Assad.

"He was one of the people pushing hardest for sanctions to be lifted against Iran.

"I do not see how that makes him a friend of the Jews and Israel. Supporting Iran and Syria - I would say that makes him an enemy of Israel. Just because he has attended a synagogue, that does not make him a friend of Jewish people."

Ever since his extradition, Browder has spoken out against Putin to every media outlet, government contact or lawmaker who'll listen. His office building is laden with framed newspaper cuttings, taken from interviews he has given to publications in the UK and elsewhere including US and Asia.

And his campaign has been successful. In 2012, the US Congress passed the Magnitsky Law which blocked 18 officials implicated in his murder from entering the US or using US banking. In response, Putin ended the practice of adoption of Russian babies by American couples.

In 2013, a Russian judge sentenced Browder to nine years in jail in absentia and a dead Magnitsky was posthumously convicted of tax evasion.

"There are now criminal cases going on in more than a dozen different countries in relation to the money laundering of proceeds of the crime that Sergei had uncovered," he adds. "I do believe it will bring a positive change in the world."

But irritating one of the most dangerous men in the world has come at a personal cost.

Browder says countless threats have been made against his life. He takes precautions, but for obvious reasons does not disclose what they are.

"The Russian government has threatened to kill me. They're trying very hard to arrest me, to get me back to Russia so they can torture and kill me in prison. There have been kidnapping threats, lawsuits, arrest warrants and Interpol arrest notices."

He reels this off calmly, almost too calmly. "When you are doing this for as long as I have, you become inured to the fear," he explains. "As a famous Mexican revolutionary once said: 'I would rather die standing up than live my life on my knees'. I won't live my life cowering in fear."

Browder has a revolutionary air; which is unsurprising given his background. His grandfather Earl Browder was the leader of the American Communist Party and twice ran for president. But, as a teenager, Browder chose a unique route to rebel against his parents.

"I figured out that the best way to rebel from a family of communists was to put on a suit and tie and become a capitalist! For many years, my parents had a hard time with that," laughs Browder, whose company was at one time the best performing investment fund in the world.

He adds: "I was never driven by the money. In my family, everyone is very accomplished in whatever their profession.

My grandfather was leader of the Communist Party. My father [Felix] was a mathematician and winner of the National Medal of Science in the United States.

"So when I went into finance, my goal was to be the best in that field."

But now, that world has been put behind him. "Business was fun and interesting but it doesn't create the same moral satisfaction as fighting for justice," he says.

Browder now lives in London. He married his first wife in Marble Arch Synagogue and his son was barmitzvahed; a ceremony he never had.

"I am a British citizen with an American background," he concludes. "I feel a mixture of everything. I've lived half my life in America and half here. My accent and many mannerisms are American, but when I'm there I also feel foreign."

It's hard to know what Browder will do next. He rules out any government ambitions, instead saying he can achieve more by lobbying it.

This summer, he says he met "big Hollywood players" in a bid to turn his book into a major film.

"The most important next step in the campaign is to adapt the book into a Hollywood feature film," he says. "I have been approached by many film-makers and spent part of the summer in LA meeting with screenwriters, producers and directors to figure out what the best constellation of players will be on this.

"There are a lot of people looking at it. It's still difficult to say who we will end up choosing. There are many interesting options, but I'm not going to name any names.

"This is a classic Hollywood good versus evil tale, the true story of one man's fight for justice - and there's nothing more compelling getting some justice in a highly unjust world.

"Putin is going to go crazy when this film gets made."

Red Notice: How I became Putin's No 1 enemy by Bill Browder, £18.99