Human rights lawyer and Revolutionary Socialists member Mahinour El-Masry was released on Saturday after being briefly detained late on Friday.
Prosecutors who examined El-Masry early on Saturday decided to open an investigation into letters she had with her at the time of her arrest, journalist and activist Rasha Azab told Ahram Online.
The letters, Azab says, were from El-Masry's fellow activists who are currently in detention – Loai Mohamed Abdel-Rahman, Omar Abdel-Aziz Hussein and Islam Mohamed Ahmed – and contained details of their conditions in prison.
However, the prosecution has since terminated the investigation after El-Masry's lawyers protested that it's not illegal to have such letters.
Her arrest on Friday came while shopping in her local market, Revolutionary Socialist member Taher Mokhtar told Ahram Online.
Mokhtar claims that there's a personal dispute between El-Masry and the police officer who arrested her.
In January, El-Masry was sentenced in absentia to two years in jail and a LE50,000 fine for organising an unauthorised protest.
El-Masry has filed an appeal against her prison sentence, allowing her to avoid detention for the time being. The court is set to review her appeal on 20 May.
El-Masry's arrest is the latest prosecution of non-Islamist activists under the country's new protest law, which bans demonstrations not pre-approved by authorities.
On 22 December 2013, a Cairo criminal court sentenced April 6 Youth Movement founders Ahmed Maher and Mohamed Adel along with independent activist Ahmed Douma to three years in jail and fines of LE50,000 each for organising an unauthorised protest and attacking security forces outside an Abdeen court the previous month.
The interim government has repeatedly defended the protest law, passed in November of last year, confirming that it is essential to restore control and stability to the streets.
Under the law, protest organisers are required to notify the authorities at least three day prior to any public demonstration. Violators can be punished with jail terms and hefty fines.