The TV satirist Bassem Youssef is among at least 20 well-known opposition figures detained last week by the Egyptian authorities in what campaigners say amounts to the most serious crackdown on free speech since the military dictatorship that ended last summer.

The crackdown started on Monday, when five leading activists were summoned for questioning by the prosecutor-general for allegedly inciting violence against the Muslim Brotherhood.Two of the most prominent were Alaa Abdel-Fattah and Mona Seif, siblings prominent during the 2011 uprising that toppled Hosni Mubarak. After no evidence emerged to support the initial charges, they were accused on Thursday of attacking the offices of a former prime minister – an incident that happened nearly a year ago.

Abdel-Fattah was previously imprisoned under the military regime that followed Mubarak's. As he arrived for questioning this week, he symbolically wore an all-white outfit similar to the one he was forced to wear during his jail term in 2011.

Elsewhere, former opposition MP Hamdy al-Fakhrany was charged with inciting violence in Mahalla, northern Egypt. Then on Friday evening, activists and lawyers protesting outside a police station in Alexandria were attacked, sexually harassed and detained by policemen, whose willingness to go after lawyers is a sign – observers say – that they once again feel untouchable as an institution.

Among those arrested was lawyer Mahinour el-Masry, a hero of the 2011 uprising. "For whoever doesn't know Mahinour al-Masry, she is one of Alexandria's bravest revolutionaries," said former MP Bassem Kamel.

In this week's most bizarre incident, four members of the 6 April protest movement were arrested after allegedly holding up women's underwear at a demonstration outside the interior minister's house in Cairo.