Madonna, Sting, and Jarvis Cocker are among the celebrities urging Russian president to release the three women.
Russian president Vladimir Putin says Russian punk trio Pussy Riot should not face tough sentencing for their pro-gay protest in Moscow.
The female musicians are on trial over hooliganism charges after a video of their anti-Putin song appeared online.
Maria Alekhina, Ekaterina Samutsevich and Nadezhda Tolokonnikova were accused on the grounds of ‘religious hatred' for singing a protest song in Moscow’s main Orthodox church.
After meeting with UK Prime Minister David Cameron yesterday (2 August), Putin told London reporters: ‘There is nothing good in what they did [but] I don’t think they should be judged too severely.’
The musicians have pleaded not guilty to the charges in the trial which began Monday (30 July).The women could face up to seven years in prison if they are pronounced guilty.
Alekhina, Samutsevich and Tolokonnikova performed the song 'Virgin Mary redeem us of Putin', and included a lyric reference to the country's persecuted LGBT community with the line 'Gay pride sent to Siberia in chains'.
It was one of a number of performances intended as a protest against Putin in the run-up to Russia’s presidential elections in March.
The song outraged the Orthodox Church in Russia, which accused Pussy Riot of blasphemy and asked for severe judgment.
Human rights groups have condemned the trial, with Amnesty International accusing it of being 'politically motivated'.
'This trial should never have taken place,' said Amnesty Europe and Central Asia director John Dalhuisen.
'The three singers are being tried for making a legitimate protest. This is not a criminal offence. They must be released immediately.
'They dared to attack the two pillars of modern Russian establishment - the Kremlin and the Orthodox Church.'
He added the expected sentence was 'wildly out of proportion'.
British musicians Jarvis Cocker, Pete Townshend, Martha Wainwright and Neil Tennant have written a letter to The Times, demanding the girls have a fair hearing.
It said: ‘We are extremely concerned about the treatment they have received.
‘We believe firmly that it is the role of the artist to make legitimate political protest and fight for freedom of speech.’
Pussy Riot has also received support from stas such as Madonna, Danny DeVito, Sting and the Red Hot Chili Peppers.
Russian authorities arrested Alekhina and Tolokonnikova on 4 March, and Samutsevich on 15 March, claiming they were masked singers in the video.
Amnesty considers the three women as prisoners of conscience and has launched a text campaign calling on the Russian authorities to drop the charges against the three women and release them immediately. You can text your support here.