Temporary Police Superintendent Emma Bond said there are two elementary schools near the vacant lot where the device was found in a residential neighborhood, the Belfast Telegraph reported.
"Those who are behind this attack had very clear and I believe murderous intent," she said. "This is a very reckless attack."
Investigators believe the mortar was placed by dissident republicans for a planned attack on the police station.
A taxi driver and a police officer who passed the lot on his way to work both spotted the device and reported it independently just after 7 a.m., police said. Investigators say it was probably driven to the area and placed there overnight.
Both schools were closed down and some houses in the area evacuated.
Tensions have risen in Northern Ireland in recent months. A vote by the Belfast City Council in December to limit the flying of the British flag at City Hall sparked weeks of violent protests by unionists, while dissident republicans have launched a number of attacks.