By Heba Saleh
BBC News, 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.
'No guarantees'
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.
Egyptian  and government supporters have dispersed Saturday demonstration against President Hosni Mubarak's fifth re-election campaign.

Several hundred demonstrators from , a movement against re-election of Mr Mubarak, as well as from other pro-reform organisations were to gather in Cairo's main square, but they were attacked by the  and  on their way.

Eyewitnesses reported that the protesters were beaten, kicked in the head, and finally thrown into police trucks.

19 members of Kifaya were arrested and they are still held, according to the organisation statement.  , Mr Mubarak's most serious opponent in the upcoming election was among those arrested, but is now freed.

Mr Mubarak announced that he would stand in the election on Thursday. Under international pressure he introduced several serious , including first in 24 years presidential elections with more than one candidate. However, opposition leaders say that is 'no guarantee' the elections will be fair.