The mob attacked the ruling National Democratic Party headquarters in the city of El-Arish and burned pictures of President Hosni Mubarak, prompting police to fire tear gas to break up the crowd, the source said.

What began as a fight between young members of two rival clans in El-Arish spilled over into large scale violence against the police, residents said.

"Police have detained 40 Bedouins from both tribes. The situation is calm now but three police officers were injured in clashes," the security source said on condition of anonymity.

At least 20 Bedouins needed medical treatment following the clashes, mainly from the effects of the tear gas, the source said.

Residents said the violence erupted overnight Saturday, when several thousand Bedouins went on the rampage in the streets of El-Arish, directing their anger against police after a gang fight involving members of the Tarabin tribe based in central Sinai and the local Fawakhriya tribe.

Witnesses said earlier that the city's local council building had been damaged by stone-throwing protesters who also burned tyres throughout the city.

"At least 4,000 to 5,000 residents took to the streets, they burned tyres and smashed shop windows," a police officer said of the overnight rioting.

"At least six people were injured in the clashes, including one police officer," the official said, giving the toll from the overnight violence.

After the breaking of the Ramadan fast, armed Tarabin tribesmen arrived in El-Arish in 15 trucks and began to shoot at members of the Fawakhriya tribe for about 15 minutes.

Thousands of residents, furious at the lack of police protection against the violence, then poured into the streets for protests that lasted at least five hours, the police official said.

Abdel Hamid Selmy, of the Fawakhriya tribe and a member of the Egyptian parliament's upper house said that police had promised to control the situation in the future.

He said that while rivalries between neighbouring clans regularly lead to fighting, Saturday's spontaneous protest "is an expression of the frustration felt by Bedouins due to the constant neglect by authorities."

Hassan Abdallah, a local resident who belongs to the left-leaning Tagammu party, said that protesters had blocked main roads and shouted anti-government slogans.

Bedouins have long complained of discriminatory policies and mistreatment by the authorities.

A spate of bombings to hit popular tourist destinations in Sinai between 2004 and 2006 lead to massive sweeps of the peninsula with thousands of Bedouins arrested. Many of them have yet to be released.

In January, a report by the Brussels-based International Crisis Group (ICG) said that discriminatory policies against the population of Egypt's Sinai risk giving rise to further terrorist activity in the peninsula.

The government "has done little or nothing to encourage participation of Sinai residents in national political life" and systematically favoured the development of other regions, the report argued.

ICG said authorities need to commit "to a new economic and social development strategy aimed clearly at benefiting the population as a whole without discrimination."