Two police officers and an onlooker were injured in the explosion. The engine was placed inside a bag that was being inspected by the police.
This happened less than two days before Coptic Christmas, a religious event celebrated by Egypt's Christians.
The Copts, who make up around 10 percent of the population, have long complained of discrimination. They have also frequently been attacked by terrorist groups who see them as infidels, prompting authorities to place armed guards outside churches and monasteries. In November, militants killed seven people returning from baptizing a child at a Coptic monastery about 260 km (160 miles) up the River Nile from Cairo.
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