Police in Egypt have arrested four leading members of the opposition group, the Muslim Brotherhood.
Less than 48 hours earlier the group held rallies across the country.
A spokesman for the group, Abdul Muniem Mahmoud, said those arrested included one of its most prominent members, Essam al-Erian.
The Muslim Brotherhood is officially banned in Egypt but sometimes tolerated by the authorities. It is said to be Egypt's most popular opposition group.
Along with secular pro-democracy movements, the Muslim Brotherhood has been vocal in calling for reform since February, when Egyptian President Hosni Mubarak agreed to allow rival candidates to contest the presidency for the first time.
Mr Mahmoud told the BBC that 1,562 members of the Muslim Brotherhood had been arrested after nationwide anti-government protests on Wednesday, not the 400 announced by the Egyptian officials.
He further claimed that many had been treated violently by the security services.
Wednesday's protests ended in clashes in several towns and police fired tear gas to disperse crowds, activists said.
Reports say there have been further arrests on Friday. More than 100 brotherhood supporters were arrested as they demonstrated to demand the release of their detained colleagues.
The AFP news agency reported that a man died on Friday after inhaling tear gas fired by security forces to disperse a protesters at Mansura in the Nile Delta region.
The Muslim Brotherhood identified him as Tariq Mahdi Ghanam, a 40-year-old school teacher.
An Interior ministry official confirmed the death, but said the man had died of a heart attack.