Moscow (AFP) - Mothers of Russian prisoners denounced the prosecution of protesters as a "travesty of justice" as they gathered outside the offices of the presidential administration on Tuesday.

Members of a new campaigning group, Mothers against Political Repression, appealed to President Vladimir Putin to intervene in an open letter read out to journalists.

The informal movement, the first of its kind, brings together families of protesters accused of violence against police, suspected extremists and others.

They called on Putin to have the courts, investigators and the FSB security service probed over their handling of the cases.

"The travesty of justice and lawlessness that are being perpetrated in front of the whole country undermine people's trust in the justice system and the state," the letter signed by 16 people said.

"In our view this negatively affects our country's stability and prosperity and leads to public discord," it added.

One woman at the gathering of about a dozen people cried as others tried to console her.

After the relatives delivered what they called "the letter of despair" to the presidential administration they held one-person pickets.

It is the only form of protest in Russia that does not require prior approval from authorities.

Putin's spokesman Dmitry Peskov said it was not the president's job to oversee the work of courts or investigators, adding the mothers' letter might be forwarded to the General Prosecutor's Office.

- 'They will have to listen' -

Yulia Pavlikova, the mother of 19-year-old Anna Pavlikova suspected of extremism, said she did not hold out much hope that her daughter would be acquitted.

She warned however that their movement would grow.

"A lot of mothers are knocking on our door in relation to other cases," she told AFP.

"And when there are many of us, they will have to listen to us."

The group was set up last month in response to a crackdown on the opposition to campaign on behalf of those convicted or detained.

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Galina Martintsova, whose 26-year-old son Maksim faces up to five years in prison for alleged violence against police, said the letter was their "last hope".

"It's clear that these are trumped-up cases," she said, praising the group for helping her to get through the ordeal.

"It's hard to fight against injustice when you are alone," she said.

Tens of thousands of people rallied in Moscow this summer after allies of opposition leader Alexei Navalny were barred from standing for seats in city parliament in September.

The authorities unleashed a major crackdown, arresting hundreds and jailing several people.

Top rights group Memorial has recognised 314 people as "political prisoners" in Russia although it warns that the overall numbers of such prisoners are higher.