The protest in the Mediterranean port city of Alexandria attracted about 150 participants and lasted only 10 minutes, according to the two activists, Ranwa Youssef and attorney Mohammed Ramadan.
The rally was in support of Mahienour el-Masry, a rights lawyer convicted earlier this year of breaking a controversial law that bans public demonstrations without advance police approval. She was sentenced to two years in prison.
The police attacked the protesters just as today's gathering was ending and later stormed offices of a volunteer agency nearby where many of the activists had met earlier, said Youssef and Ramadan.
The convicted activist, el-Masry, has joined several icons of Egypt's 2011 uprising that toppled longtime autocrat Hosni Mubarak who have been imprisoned under the controversial law on demonstrations, adopted late last year.
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Egypt's retired army chief, Abdel-Fattah el-Sissi who is the front-runner in next week's presidential elections, says he will keep the law if elected in the May 26-27 vote.
At the same time, there has been a massive crackdown against the Muslim Brotherhood, the Islamist group from which Mubarak's successor, Mohammed Morsi, hails. The Islamist Morsi was ousted by el-Sissi last July.
At least 16,000 Brotherhood members and allies have been jailed and hundreds have been killed during protests since July. Morsi and most other senior group leaders are in detention.
El-Masry had remained free until Tuesday, when her conviction and sentence were upheld. She was also fined 50,000 pounds (nearly USD 7,000).