Clashes in Cairo between Islamist protesters and police killed two people Friday, just hours after assailants shot and killed two senior Egyptian army officers.
Egyptian security sources said fighting between protesters and police broke out in the Matariya neighbordhood in east Cairo, following calls for action by Islamists who want to topple Egypt's military-backed government.
At least 100 people were arrested.
The ultra-conservative Salafi Front, which campaigns for the imposition of Islamic law throughout Egypt, called for a "Muslim Youth Uprising" Friday with nationwide demonstrations.
The Muslim Brotherhood endorsed the call for mass protests.
Few details were available about the killings of two senior army officers. They were gunned down in two separate incidents, in Cairo and just to the north of the city.
Friday's protests appeared to be the most concerted effort in months to rally opposition against the government of President Abdel-Fattah el-Sissi.
Authorities increased security at key locations and warned there would be a tough response to any violence.
Sissi, formerly chief of the army, led last year's military action that ousted President Mohamed Morsi, a Muslim Brotherhood leader who was Egypt's first elected president.
Since Morsi's overthrow, the government has jailed tens of thousands of Muslim Brotherhood members and supporters.
Hundreds have also been killed in violence that often breaks out during protests, though the protests themselves have been dwindling.