“There was no security at the gate,” one woman told reporters. “They were all having breakfast inside their van.”

A man asked, “You’re coming now after everything was destroyed?”

There was no immediate claim of responsibility, although the attack bore the hallmark of Islamist militants fighting President Abdel Fattah el-Sisi who have previously targeted minority Christians over their perceived support for his government.

It was the second major attack in the Egyptian capital in three days, marking a jarring return to violence after months of relative calm. An Islamist militant group claimed responsibility for an explosion at a security check post on Friday that killed six police officers.

Mr. Sisi’s strongman rule has come under economic pressure in recent months amid high inflation and a sharp drop in the value of the Egyptian pound. Threatened street protests last month did not materialize, but the surging attacks may be an attempt to stoke opposition through violence.

Egyptian security officials, quoted by state media, said that an explosive device containing about 26 pounds of TNT had been placed in the chapel. It went off during Mass around 10 a.m.