A large riot broke out in the Saudi city of Madinah on Thursday night. Arab News says:

According to some eyewitnesses, the trouble started when some teenagers and youths in al-Osbah neighborhood and residents of nearby districts began throwing stones at one another at about 10 p.m. on Thursday. The rioters, whose number he estimated at 800 men, also broke window glasses of parked cars.

Police have not said what caused the trouble and according to Arab News they are still trying to find out.

Posted by Brian Whitaker, 18 Dec 2010.

UPDATE, 18 December: AFP and AP are reporting the clashes as a conflict between Sunni Muslims and the minority Shia Muslims who were taking part in Ashoura commemorations. The rasid.com website also has a report in Arabic.