Madonna introduces Maria Alyokhina and Nadezhda Tolokonnikova at an Amnesty International concert in New York. Photograph: Jason Szenes/EPA
Members of the Russian punk collective Pussy Riot have accused their previously imprisoned bandmates of undermining the group's ideals by appearing at a charity concert introduced by Madonna.
A letter posted on the group's blog lambasted Nadezhda Tolokonnikova and Maria Alyokhina for taking part in the concert organised by Amnesty International in New York on Wednesday night.
Madonna introduced the pair, who were dressed in tunics with crucifixes emblazoned on the front. They then delivered a scathing attack about the regime of the Russian president, Vladimir Putin, chanting: "Russia will be free!" to the audience at the Barclays Centre in Brooklyn.
But their appearance was poorly received by others in the punk collective, which has a fluctuating membership that has never been fully revealed.
Selling concert tickets "is highly contradictory to the principles of Pussy Riot", said the letter signed with six nicknames: Cat, Garadzha, Fara, Shayba, Serafima and Shumakher.
"We're a female separatist collective," it said. "We never accept money for our performances … we only stage illegal performances in unexpected public places."
The letter also took issue with concert posters showing a male guitarist in a balaclava, a trademark of the feminist group.
Tolokonnikova, 24, and Alyokhina, 25, were freed in December three months before the end of their two-year sentences for staging an anti-Putin "punk prayer" performance in a Moscow cathedral. On their release the said they would focus on campaigning for the rights of prisoners.
Wednesday's concert also included performances by the Flaming Lips and Blondie. Ticket prices started at $27 (£17).
"They have said in every interview that they have quit the group and no longer represent Pussy Riot," said the letter. "But all of their appearances are announced as appearances by Pussy Riot.
"They are no longer Pussy Riot. We have lost two friends, two ideological teammates, but the world has gained two brave rights activists," said.
Five members in colourful tights and balaclavas staged the cathedral performance in February 2012. Three were arrested the following month.
One, Yekaterina Samutsevich, was freed with a suspended sentence after a successful appeal. The other two performers have never been identified.
Since the arrests, there have been no more guerrilla Pussy Riot performances in public places, although a music video targeting Russia's oil industry appeared last July, apparently by a splinter group that had set up a new website.