Representatives from the April 6 Youth Movement and its offshoot the Democratic Font held a press conference on Tuesday in which they announced they would appeal Monday's court ruling banning the group and ordering the closure of its offices.
"We will appeal the verdict before the administrative court," said Sherif El-Hosary, head of the legal department of the Democratic Front, which split from the April 6 Youth Movement in 2011 after a dispute between the group's leaders.
El-Hosary accused the verdict of being "politicised" and from a court that lacks subject matter jurisdiction.
On Monday, Cairo's Court for Urgent Matters ordered a freeze on April 6's activities, charging the group with espionage and defamation of the state. None of the group's individual members, however, face such charges.
During Tuesday's press conference, April 6 press official Mohamed Kamal accused the current authorities of trying to silence "all serious opposition, political forces and movements", while leaving only "cartoonish opposition parties" for the purpose of propaganda.
Following Monday's ruling, April 6 released a statement insisting that the movement would "keep going with [its] activities and views the way [it] wants."
El-Hosary slammed the court evidence used against the movement.
The lawyer who filed the suit, Ashraf Saeed, said that the case was based on recordings aired by the well-known TV host Abdel-Rehim Ali – an outspoken critic of April 6 members and other prominent figures from the 25 January 2011 revolution.
Over the course of several episodes of his "Black Box" programme, Ali aired private recordings that he said belonged to activists, including April 6 co-founder Ahmed Maher, which he said was proof that they had "conspired against state institutions".
Ali claims he possesses over 5,000 such recordings.
El-Hosary, though, argued on Tuesday that Monday's court verdict was based on "leaked personal calls that were recorded and aired illegally" along with photos of Maher and April 6 figure Mohamed Adel and activist Ahmed Douma, from a 2008 trip to the Gaza Strip as part of a "solidarity" effort against the Israeli occupation of Palestine.
"This [evidence] does not incriminate the April 6 Youth Movement," said El-Hosary.
Meanwhile, April 6 members and other political forces have called for a protest on Wednesday in front of the Journalists' Syndicate in downtown Cairo to denounce the court verdict.
Earlier this month, the group marked its sixth anniversary with two of its founding members, Maher and Adel, behind bars.
Maher, Adel and Douma were sentenced last December to three years in prison and fined LE50,000 each for violating a protest law which criminalises unauthorised demonstrations and for assaulting police officers.
An appeal against the verdict was turned down on 7 April.