The Voice of Beslan group has been charged with "extremism" over an appeal to politicians in Europe and the US which implied that Putin assisted terrorists.

The prosecution was launched under legislation introduced last year which civil rights activists warned could be used to attack critics of the Kremlin.

More than 1,000 people were taken hostage during the siege in 2004, when militants seized a school in Beslan, North Ossetia, a region bordering Chechnya. The three-day stand-off with security forces ended in a bloodbath when two explosions destroyed part of the school gymnasium and provoked a gun battle. A total of 334 people died, more than half of them children.

No senior official was put on trial, despite numerous witnesses testifying that the rescue attempt was a shambles. An investigation by federal MPs found the militants were at fault for all the deaths.

Relatives of the dead and some local politicians insist security forces fired on the gymnasium before it was fully evacuated. The Voice of Beslan said in its release that Putin was "a president who solves his problems with tanks and flame-throwers". He had become a "guarantor for criminals", it added. Prosecutors, who claimed they found the appeal during routine monitoring of the internet, said the accusations were unlawful.

Voice of Beslan has led a campaign to get security service officials charged over the siege. Last year members of the group erected a sign pointing at the ruins of the Beslan school reading "Putin's Course".

Emma Tagaeva, the chairman of Voice of Beslan, described the prosecution as "an order from Moscow", in revenge for the group's struggle for justice. In a letter to Putin asking him to halt today's prosecution, she wrote: "We consider you guilty of the deaths of our children but does that represent a manifestation of extremism? Will it bring honour to the law enforcement organs to accuse the victims of a terrible tragedy of extremism?"

Human rights activists have spoken out against the trial. Lyudmila Alekseeva, head of the Moscow Helsinki Group, said: "This is all happening because Voice of Beslan is making every effort so that the investigation into the deaths of such a huge number of children and adults doesn't go quiet. That's why the might of the state, the court system and administrative methods are being used to battle against these unfortunate women who lost their children."

The law was amended in 2006 to expand the definition of extremist activity to include public slander of a government official while performing his duties. Critics said the changes would be used by the Kremlin to silence opponents. Under Russian law a person who publishes extremist literature can be jailed for up to three years.

Lev Ponomaryov, head of the For Human Rights movement, said: "If this approach to the problem of extremism is taken, then every opposition figure in Russia could be charged."