Altogether 400 people were detained on Wednesday, when thousands of supporters of the country's largest Islamist opposition group protested in Cairo, several cities in the
Delta and in Fayoum, south of Cairo.
A strong security presence prevented protests in the northern cities of Alexandria and Mansoura.
In a statement issued on Thursday, the Muslim Brotherhood said more than 1500 were arrested in the protest, the latest in a growing opposition campaign demanding political reform.
The 280 ordered to be further detained were from four provinces, prosecution officials said on condition of anonymity. They are accused of illegally gathering, obstructing traffic and being members of a banned group.
The Brotherhood statement condemned the arrests and warned of "the imminent danger and severe boiling point inside the society".
Protests against Mubarak have
"We will continue to undertake our religious and nationalist duties," it said.
The Muslim Brotherhood, established in 1928 and banned since 1954, is used to intermittent government crackdowns.
Egyptian President Hosni Mubarak said in an interview with Egyptian television last month that he would not allow any religious group to become a political party but added that he would not object to Muslim Brotherhood members joining
Meanwhile, relatives of a cousin of a suspect in the 7 April bombing in the capital, said on Thursday that he had died of torture while in police custody.
Moustafa Ahmed Abdullah said police ordered him to open the family grave on 28 April and quickly bury Mohammed Soliman Youssef, a 40-year-old teacher who was one of scores rounded up after the bombing.
Youssef was a cousin of Ashraf Saeed Youssef, 27, who is accused of being one of the masterminds of the 7 April bombing near a large tourist bazaar in which three tourists were killed.