The protesters - many from the secular pro-democracy Kefaya (Enough) movement - chanted "Illegitimate, Illegitimate".

Mr Mubarak - who has been in power for 24 years - was re-elected to office in the first-ever contested elections, which were marked by a low turnout.

He took 88.6% of votes in the ballot three weeks ago, although less than a quarter of the electorate voted.

Opposition groups dismissed the ballot as a fraud, but electoral officials have rejected calls for a re-run.

Presidential oath

Within hours of Mr Mubarak"s swearing-in ceremony - attended by several ambassadors and dignitaries, including Libyan leader General Muammar Gaddafi - hundreds of protesters took the streets.

Demonstrators were a mix of young and old, men and woman, of various political persuasions, including leftists, liberals and Islamists.

Hosni Mubarak won the first contested election by vast margin 

Police allowed them to march through the streets of downtown Cairo - a dramatic development as past demonstrations have only been tolerated provided they stay in one place, says the BBC"s Magdi Abdelhadi from the scene.

As well as opposing President Mubarak, they were also protesting against his son Gamal Mubarak who, many believe, is being groomed to succeed his father.

A protest over the election outcome on Saturday saw 2,000 people take to the streets of Cairo.

Taking office, Mr Mubarak pledged "to uphold the republican system with loyalty, to respect the constitution and the law, to look after the interests of the people fully and to safeguard the independence and territorial integrity of the motherland".

He had previously been elected only in single-candidate referendums, but changed the system earlier this year under pressure from the US and domestic political groups