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
Prince Harry will move down to sixth in line to the throne.
Ongoing problems with the bank's online platform have seen customers make a raft of complaints.
The Duchess of Cambridge has been driven to St Mary's Hospital in London in the "early stages of labour", Kensington Palace has said.