With over 600 barrios, a definitive breakdown of Managua safety is a book in itself. As a general rule, don't ever walk more than a few blocks anywhere in Managua. There are almost no police during the night time and with no centre there are few places where the streets will be busy. The Metrocentro area is safe, but it's still best not to walk alone. The only place that lends itself to walking is the malecón and central park area of old Managua, but do not walk here at night under any circumstances. Even during the daytime take precautions, don't carry any more than you need and avoid walking alone. Be careful when visiting the Catedral Nueva, which is next to a barrio with many thieves. Having said all of this, in comparison with other Central American capitals Managua is safe for the visitor, although theft is common at bus stations and outside the more affluent neighborhoods
That's from my guidebook. It seemed fine to me.