Story highlights The American Civil Liberties Union on Thursday filed a lawsuit against the city of Philadelphia

A progressive social justice group was denied a permit to protest the Democratic National Convention

(CNN) The American Civil Liberties Union on Thursday filed a lawsuit against the city of Philadelphia on behalf of the Poor People's Economic Human Rights Campaign after the progressive social justice group was denied a permit to protest the Democratic National Convention.

"We have a whole history of nonviolent, orderly and very large marches," PPEHCR national organizer Cheri Honkala told CNN. "We were devastated (by the city's decision) and had no other recourse."

An estimated 500 people are seeking to march "from City Hall to the doorstep of the DNC" under the PPEHRC banner -- part of a demonstration scheduled to coincide with the opening of the convention on July 25 at the Wells Fargo Center.

The ACLU is already representing a number of groups in litigation against Cleveland, alleging that the Republican convention host city's security procedures are set up to infringe on demonstrators' free speech protections. Both conventions, which take place on consecutive weeks beginning July 18, are expecting heavy protest activities and have come under criticism for trying to effectively cordon off the events from dissenting voices.

In the complaint delivered today, the ACLU argued that the city's "blanket ban" on rush hour marches will "violate PPEHRC's right to free expression under the First Amendment," noting that the group peacefully protested the 2000 GOP convention while walking the same route it sought for July.

Read More