By Heba Saleh
BBC News, Cairo
Egyptian security forces have used force to disperse pro-reform demonstrators trying to hold a protest in a main square in central Cairo.
Similar protesters were allowed to pass off unchallenged
Opposition activists had called a demonstration to protest against President Hosni Mubarak's decision to stand for re-election.
The demonstrators were set upon by riot police wielding sticks and clubs.
Kifaya, a movement campaigning against his re-election, said 19 of its leaders had been arrested.
Protesters were kicked and beaten up before being thrown into police vehicles waiting at the scene.
Kifaya called the protest but other opposition forces were also represented.
Mr Mubarak announced on Thursday that he would stand for re-election.
He promised political and constitutional reforms and said that, as he put it, Egyptians would be free citizens in a democratic country.
In recent weeks, Kifaya's weekly demonstrations passed off peacefully without any interference from the police.
Many had thought the authorities would be keen to avoid a repeat of the international outcry which followed assaults in May on demonstrators calling for a boycott of the referendum on constitutional reform.
Mr Mubarak has been facing unprecedented internal and external pressure to introduce democratic changes.
He has shepherded a constitutional change allowing contested presidential elections for the first time this year.
Five opposition leaders have already said they intend to run against him but the leaders of the main parties say there are no serious guarantees the process will be fair or transparent.