Egyptian authorities have arrested the former interior minister Habib el-Adly and two other former ministers who are under investigation for alleged corruption.
Authorities also arrested the steel tycoon Ahmed Ezz, once a prominent member of the ousted leader Hosni Mubarak's ruling National Democratic Party.
Mr Adly, whose job gave him control over the 500,000-strong security forces, has been widely blamed for the deadly brutality used by riot police against demonstrators in massive protests that began on January 25 and forced Mr Mubarak to step down on February 11.
Fresh demonstrations were expected yesterday in Cairo in celebration of Mr Mubarak's departure and to maintain pressure on the military regime that replaced him to press forward with reform.
News of Mr Adly's arrest followed the detention on Thursday of the former housing minister Ahmed al-Maghrabi, the former tourism minister Zuheir Garana and Mr Ezz.
All four face allegations that range from money laundering to abuse of authority and squandering state wealth.
The protesters who ousted Mr Mubarak after 18 days of demonstrations often mentioned the deep corruption of the regime as a key reason behind their movement.
Security officials, who spoke on condition of anonymity because they were not authorised to speak to the media, said all four of the men would initially be held for 15 days.
All four had previously been banned from travel abroad and had their assets frozen, measures that are normally a prelude to a criminal investigation and possible trial.
They are among about a dozen former ministers and businessmen being investigated for alleged corruption or abuse of authority.
Most of them belonged to a clique of businessmen-turned-politicians who rallied around Mr Mubarak's son and one-time heir apparent, Gamal.
Gamal Mubarak, 47, rose rapidly through the ranks of the party during the past decade to become the most powerful politician in Egypt after his 82-year-old father.
Gamal and his circle of businessmen have been blamed for orchestrating economic reform that liberalised the economy but left the country's poor masses unable to reap the benefits of economic growth.
Mr Ezz, who used his wealth to promote his political career, is widely blamed for the fraud that marred parliamentary elections held in November and December. The ruling party won all but a small fraction of the chamber's 518 seats.
Mr Ezz denied the charge in a television interview.
Mr Adly served in his former post for 12 years.
The BBC reported that organisers predicted a million people would turn out for yesterday's demonstration.
''I am here now to monitor how the military is going to take things,'' said Nasser Abdel-Hamid, a 28-year-old information systems engineer. He is a member of the representative body of the Coalition of the Youth of the January 25 Revolution, the main grouping of the activist organisers.
The United States gave Egypt $US150 million in economic assistance on Thursday, saying it would help its ally move towards democracy.
Associated Press, Agence France-Presse