Egyptian police have arrested 28 alleged members of the banned Muslim Brotherhood.
Those arrested were allegedly planning a demonstration in Cairo's al-Azhar mosque to "stir the public against the regime", according to police sources.
The Egyptian Government has accused the Brotherhood - one of three main Islamic groups in Egypt - of encouraging Islamist-inspired violence; the Brotherhood insists it is seeking to establish an Islamic state using peaceful means.
The planned demonstration was in support of about 120 people arrested in Alexandria at the end of June on suspicion of belonging to the organisation and causing a disturbance, sources said.
Those detained in Alexandria accused the government of preventing them from casting votes in a by-election contested by two Muslim Brotherhood candidates.
Candidates from the ruling National Democratic Party won the seats.
Former President Gamal Abdel Nasser banned the Muslim Brotherhood after an attempt on his life in 1954.
Thousands of its members were held without trial over a 20-year period, and some of its leaders executed.
Since 1995, hundreds of Brotherhood members have been tried and scores imprisoned.
The arrests follow a call for an end to violence by leaders of one of Egypt's other Islamic groups, Gamaa Islamiya.