The Craigavon Bridge was closed in the alert
Hijackers who forced a driver to abandon his van on a Londonderry bridge put lives at risk and cost time and money, a senior police officer said.
Chief Superintendent Stephen Martin said the gang put objects into the van and forced the driver to abandon it on the lower deck of Craigavon Bridge.
That deck remains closed following a controlled explosion at the scene.
"This is an attack on the whole community, putting lives at risk," Chief Superintendent Martin said.
"The circumstances are extremely challenging in this instance, not least because of the location of the van.
"In addition to ensuring the safety of the community, police and army technical officers, we are doing our utmost to protect the integrity of the bridge."
Jim Roddy, manager of the City Centre Initiative, said the alert which happened on Friday evening but which meant the bridge was closed for most of Saturday, had a "devastating impact" on traders.
"Whoever has done this is no friend of the city. The only people they are harming is the people of the city," he said.
Meanwhile, a security alert in Strabane has ended. The army found a "viable device" at the town's police station.
A security alert at Strabane police station has ended
The roads around the Agharan crossroad on the main Dungannon to Cookstown Road have also reopened after a suspicious object was declared a hoax.
A series of security alerts on Friday caused traffic disruption across NI.
An alert in Donegall Lane in Belfast city centre was a deliberate hoax, while alerts at the Cityside retail complex in north Belfast and at Central Station ended on Friday evening.
Army bomb experts called to the scenes found nothing suspicious.