The protest was the first to be forcibly dispersed in accordance with a controversial protest law introduced in the same month, which criminalises unauthorised public demonstrations.Activists took to social media to decry the decision, the latest in an unprecedented crackdown on dissent under Sisi, a former general who led the military overthrow of his elected but divisive Islamist predecessor Mohammed Morsi in 2013.
Thousands have been imprisoned, with some rights advocates putting the number as high as 60,000.
"I'm sorry. Our hearts are with everyone who's been following us and rooting for Alaa and justice," said Abdel-Fattah's aunt, Ahdaf Soueif, a novelist and rights advocate.
An outspoken blogger, Abdel-Fattah has been in and out of prison in the years since the 2011 ouster of autocrat Hosni Mubarak.