Leading British musicians, including Pete Townsend of The Who, Alex Kapranos of Franz Ferdin and Neil Tennant of the Pet Shop Boys, are calling on Russia to free three members of the punk rock band Pussy Riot, who are on trial in Moscow on hooliganism charges.

In an open letter published in The Times of London Thursday, the musicians said the charge was “preposterous,” arguing that artists “make legitimate political protest and fight for freedom of speech.”

The letter was published as Russian President Vladimir Putin traveled to London for talks with Britain's Prime Minister. Following the meeting, Mr. Cameron said he raised the issue of the band with Mr. Putin as part of wider discussion on human rights.

The Russian leader has faced facing growing criticism for trying to silence dissent after members of Pussy Riot went on trial and a prominent opposition blogger, Alexei Navalny, was charged with stealing state funds.

In another development Thursday, Russia's Justice Ministry officially registered a new anti-Putin party comprising of a coalition of opposition groups.

The RPR-PARNAS political party brings together the People's Freedom Party founded by Mr. Putin's former Prime Minister Mikhail Kasyanov and the Russian Republican Party headed by opposition lawmaker Vladimir Ryzhkov.

The new party could pave the way for some of the country's opposition leaders to run in future elections.