Al-Gamaa Al-Islamiya's Building and Development Party released a statement late Sunday asserting that a protester killed in Upper Egypt's Beni Suef governorate earlier in the day was a member of the Islamist group.
One man was declared dead following clashes between supporters and opponents of President Mohamed Morsi near the Freedom and Justice Party's offices in Beni Suef.
Atef Marzouk, a leading member of the group in Beni Suef, accused opponents of President Morsi of initiating the attack by firing birdshot at a pro-Morsi march.
"We defended ourselves until one of us was martyred," added Marzouk.
Hours earlier, Ibrahim Hodeib, chief of security in Beni Suef, stated that the slain man had been an anti-Morsi protester.
Hodeib also accused Al-Gamaa Al-Islamiya member Ahmed Youssef of being behind the attack.
Journalist Shaimaa Mafhouz, present during the attack, told Ahram Online that a thousand-strong anti-Morsi rally in El-Modereya Square – the main square in Muslim Brotherhood stronghold Beni Suef – was fired on.
"A number of assailants, accused by protesters of being Islamist supporters of Morsi, attacked the rally," she said. "Some clusters of protesters scattered and hid inside the mosques surrounding the square; at the same time the armed forces mobilised to contain the situation."
"Assailants continued briefly to fire at the mosques where protesters were hiding, but they ran away when the army arrived," said Mahfouz, who said she saw a child injured by a bullet to the shoulder.
The Egyptian Popular Current, led by Nasserist activist Hamdeen Sabbahi, further accused "30 members of the Jihadist movement in Beni Suef" of firing on protesters.
Sunday, which remained peaceful for most of the day, witnessed clashes at night in the Upper Egyptian governorates of Beni Suef, Assiut and Fayoum.
In Assiut, at least four people were killed while taking part in an anti-Morsi protest of thousands, which was attacked by unknown assailants as they marched near the Freedom and Justice Party’s headquarters in the Upper Egyptian governorate.