Egypt has warned users of social media such as Facebook and Twitter that they could be arrested for inciting violence through their posts, which were being tracked by the state.
Ten members of the Muslim Brotherhood, recently declared a terrorist organisation by the Egyptian government, have already been arrested for such acts, the Interior Ministry said.
Egyptian authorities were using "modern technology to track" those inciting violence against police and civilians or posting information on how to make explosives, the ministry said.
Since the ousting of President Mohamed Morsi by the army last July, the new military-installed authorities have cracked down on his supporters and the Brotherhood.
But the new arrests, which took place on Wednesday and Thursday, were the first to target a group for allegedly administering social media pages.
The allegations concern dozens of Facebook pages that authorities say called for protests against the government and denounced the police. Some of the pages were allegedly set up within the past two weeks.
In a statement, the Interior Ministry accused those arrested of using social media to "incite violence, target citizens, make bombs and carry threatening messages".
Social media and Facebook in particular were key platforms for organising the country's 2011 uprising that led to the fall of longtime President Hosni Mubarak.
Since then, fiercely anti-police pages have arisen during subsequent years of turmoil, created by youths of various stripes, including riotous football fans who often clash with authorities.
Of the Facebook pages investigated by Egyptian authorities, one contained pictures of a military officer declared to be "under the microscope," while another had a posting that called for the burning of police stations.
Others included pictures of youth throwing firebombs and instructions on how to make paint-filled eggs to throw at the windshields of police vehicles.
Egyptian authorities have arrested thousands of Brotherhood members and hundreds more have died in clashes since army chief Abdel Fattah el-Sisi toppled Morsi on July 3.
The Brotherhood has held non-stop protests demanding Morsi's reinstatement.