Const. Pierre Bourdages of Halifax Regional Police said a driver found a firearm on his bus at about 11 a.m., more than two hours after the initial sighting of someone allegedly concealing a weapon nearby.
In a statement, Bourdages said police are investigating and it was not clear if the two events are related. He also did not know what type of gun was seized.
"We can confirm that there were no threats made to anyone, no shots fired and no reports that an armed man entered any building," the statement says.
Police began searching downtown at about 8:30 a.m. when they received a single report of a man with an earpiece walking with a gun concealed under fabric on Duke and Brunswick streets, adjacent to the street where the gun was later found.
At 11 a.m., Bourdages said they arrested a 25-year-old man without incident after he allegedly left a gun on the bus and walked down Argyle Street close to where the caller reported seeing someone with a gun.
He said the man remained in custody while the investigation continues, but no charges had been laid.
Some schools in the area responded by restricting the movement of students and limiting access to the buildings.
Doug Hadley of the Halifax Regional School Board said police advised Citadel High School to go into a hold-and-secure mode, which meant about 1,200 students could move freely inside the building but no one was allowed to leave or enter it. Three elementary schools in the area also restricted students' movement, with some keeping children inside at recess.
The Capital District Health Authority heightened security at all of its medical sites, but appointments continued.
A number of public buildings downtown, including museums and the public archives, closed Thursday morning but reopened shortly after the arrest was made. Access to Halifax City Hall was also restricted.
Premier Stephen McNeil said after the fatal shooting of a soldier at the National War Memorial in Ottawa on Wednesday and the arrest in Halifax, he's asked his deputy minister to lead a committee to examine the security of all provincial government buildings.
"We need to make sure that people who come to work for the province of Nova Scotia can feel safe and secure any day," said McNeil.
The review will be conducted over the next few months and look at how quickly and effectively buildings can be locked down, among other things.
The Nova Scotia legislature was closed to the public for a second day in a row Thursday and only people with official passes had access to the building.
New Democrat MP Megan Leslie commended the work of the police in Halifax a day after the soldier was killed in Ottawa.
"After yesterday's tragic and traumatic events in Ottawa, this news is especially disturbing," Leslie, who represents Halifax in the House of Commons, said in a statement. "However, it demonstrates that our law enforcement and first responders are on high alert across our country, and are ready to take swift action in order to protect the safety of Canadians."
Bourdages cautioned that investigators had not found any link between the events in Halifax and the shooting in Ottawa and another incident in Quebec where a Canadian Forces member died in a hit-and-run attack.
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