Map: Londonderry, Northern Ireland

(CNN) -- The discovery of a homemade mortar and four live shells in a van with the roof cut open led to the arrest of three suspected Irish Republican Army dissidents, police in Londonderry, Northern Ireland, said Monday.

The men were on their way to target a police station in an urban neighborhood when police intercepted them Sunday night, said Chief Superintendent Stephen Cargin, Londonderry district commander for the Police Service of Northern Ireland.

Two were arrested at the time. Police tracked down a third man later, Cargin said.

"I have no doubt the capability of those devices was nothing more than mass casualties, potentially mass murder," he said.

After discovering the shells, which Cargin described as "crude, homemade devices," police evacuated more than 100 homes while being attacked by youths throwing objects, including gas bombs, Cargin said.

Had the shells been fired, they could have hit residential areas surrounding the police station, injuring civilians instead of police officers, Cargin said, describing the would-be attackers as "absolutely reckless."

"They were not only putting police officers' lives at risk, but they were recklessly putting the lives of their own community at risk," he said.

Cargin described the plot as "a reckless attack by dissident Republicans to murder police officers in the city of Derry."

"Dissident Republicans" refers to Irish nationalists who disagree with the 1998 peace accords aimed at ending decades of largely sectarian violence in Northern Ireland and Britain.

The conflict pitted a predominantly Protestant faction that wanted Northern Ireland to remain part of the United Kingdom against Catholics who wanted it to become part of a united Ireland.

"To characterize these people, they are voices from the past and there's no future for them," Cargin said.

Londonderry has been wracked with violence in the past.

Last year, two bombs exploded in the city -- one near government offices and police headquarters, the other near a tourism office.

It was also the site of the infamous 1972 "Bloody Sunday" incident in which British soldiers fired on civil rights marchers, killing 13.

CNN's Michael Pearson contributed to this report.