Russian prosecutors on Monday accused Bill Browder, a prominent critic of Vladimir Putin, of ordering the murder of lawyer Sergei Magnitsky and several other business associates - the latest in a string of efforts to discredit one of the most vocal advocates of sanctions against the regime.

Magnitsky, a lawyer working for Mr Browder, was arrested in 2008 after uncovering evidence of large-scale tax fraud among Russian officials. He died a year later in a Moscow prison, after he was reportedly assaulted and denied proper medical treatment. Mr Browder, once an early supporter of Mr Putin, has since lobbied tirelessly for sanctions against the Russian president and his entourage.

Mr Browder, the British head of investment fund Hermitage Capital Management, branded the accusations "Kafka-esque". Writing on social media, he cast them as a vendetta waged by Mr Putin for his lobbying, which in 2012 led to US sanctions on Russia in what is widely known as the Magnitsky Act.

"I really struck a nerve with the Magnitsky Act," Mr Browder said.

The accusations come as Interpol, an international body for coordinating national police efforts, meets this week to elect a new president. A report published by The Sunday Times said a Russian Interior Ministry official is expected to win, sparking concern that Moscow may use the organisation to target critics and dissidents residing beyond Russian police jurisdiction.