CAIRO (AP) - The Egyptian authorities have arrested the brother of Al-Qaeda chief Ayman al-Zawahri, a security official said Saturday.

He said Mohammed al-Zawahri, leader of the ultraconservative Jihadi Salafist group, was detained at a checkpoint in Giza, the city across the Nile from Cairo.

Mohammed al-Zawahri's group espouses a hardline ideology but was not clandestine prior to Egypt's July 3 coup. He was allied with ousted President Mohammed Mursi, an Islamist, whose supporters are now taking to the streets to protest the killings of its supporters in a security crackdown last week.

The official declined to give further details. He spoke anonymously as he was not authorized to talk to the press.

The authorities said earlier that al-Zawahri had commanded insurgents in the Sinai Peninsula.

Street fighting in Cairo and other clashes across the country left 173 people dead on Friday, the authorities say.

An anti-military coalition led by Mr Mursi's Muslim Brotherhood has called for a week of protests, further escalating unrest in the country. The coalition says that they will not back down until it topples the government installed by the military.

Clashes continued on Saturday morning near a downtown Cairo mosque, where hundreds of Mr Mursi's supporters remained after barricading themselves inside overnight.