A 15-year-old member of Egypt's Muslim Brotherhood was killed and around 60 people wereinjured in an attack on its headquarters in Damanhour, according to the Brotherhood's website.
"Brotherhood member, Islam Fathy Masoud, 15, was killed and 60 were injured after thugs attacked the Muslim Brotherhood's headquarters in Damanhour in the total absence of police forces," Reuters cited the website as saying.
The clashes in Damanhour came in the wake of other demonstrations across Egypt, both supporting and condemning President Mohamed Mursi's constitutional decree granting himself sweeping powers.
Police in riot gear used tear gas in an attempt to disperse protesters after the clashes broke out.
More top news
Great British Bake Off star Sue Perkins reveals she's been living with a brain tumour for eight years.
After a day where thousands of migrants tried to make their way to Germany, tonight they are sleeping on Budapest's streets.
After a bit of a nippy feel over night and an Autumnal feel into the morning.