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 cried as others tried to console her.
Mothers Against Political Repression was set up last month in response to a police crackdown on the opposition to campaign on behalf of those convicted or detained.
One member of the movement, rights activist Alexandra Krylenkova, described their message to Putin as a “letter of despair”.
“So many of these cases have resulted in monstrous sentences,” she told AFP.
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 told AFP.
Tens of thousands of people rallied in Moscow this past summer after allies of opposition leader Alexei Navalny were barred from standing for seats in city parliament in the September elections.
The authorities unleashed a major crackdown, arresting hundreds and jailing several people.
Several groups, from priests to teachers, have also signed open letters in support of the victims of what they argue was a disproportionate and arbitrary clampdown.
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