Who is Bill Browder?
An American-born, British businessman, who describes himself, not entirely unfairly, as Putins number one enemy.
Mr Browder was running an investment fund in Russia in the late nineties and early 2000s, which he says uncovered corruption in various Russian state backed commodities companies.
After Browder left Russia in 2005 his lawyer, Sergei Magnitsky, was tortured and died in Russian police custody in 2009.
Browder has since dedicated his life to fighting corruption among Vladimir Putin and his inner circle.
The allegations made by Magnitsky concern the extraordinary web of corruption that created Russias oligarch class, when huge state assets in oil and gas were sold off for tiny fractions of their overall worth then sold back to the Russian state in the following decades for billions.
Why was he arrested?
Browder has been refused entry to Russia since 2005, and described as a threat to national security. Russian authorities have even accused him of being responsible for the death ofSergeiMagnitskyhimself.
He was detained briefly via a request from Interpol on Wednesday morning, but Spanish police have since been instructed by Interpol not to honour the arrest warrant put out on Mr Browder by Russia
Why does Russia consider him such a threat?
For many years, Mr Browder and others have been lobbying Western countries like the US and the UK to bring in what he calls a Magnitsky Act. The US did so under Obama, and the UK following suit although so far to a lesser extent.
Mr Browders firmly held view is that western countries have huge power and leverage in stamping out corruption by Putin and his cronies by seizing wealth assets that they keep in western banks, and especially London property.
He also claims Putin has a personal fortune of tens, and possibly hundreds of billions of dollars kept abroad in the name of willing acolytes, and that the US Magnitsky Act has the remove it all from him.
It is for this reason he says Putin is so concerned by his actions.
How come he’s British?
He was born in the US. Hisfather Felix Browder even won various national medals for mathematics. He sayshe renounced his US citizenship as a consequence of his family having been viciously persecuted in the 1950s, in the McCarthy era. But it also meant he did not have to pay US tax on his substantial foreign earnings while he ran his investment firm in Russia.
He has described his life now as one of living in fear. In the wake of the Salisbury poisonings, Browder told The Independent the poisonings would have been carried out on Putins instructions, and done to warn his potential opponents that nowhere is safe.
He says he does not employ bodyguards, and he said it would make him less safe not moreso.