There is a "strong possibility" that dissident republicans will carry out attacks in the run-up to Christmas, a senior police chief has warned.
Assistant Chief Constable Will Kerr said violence from such groups posed a "severe threat" in Northern Ireland.
"They are dangerous, they could do real harm," he said.
As a result, security checkpoints will return to the streets of Belfast and other places where the threat level is considered to be high.
ACC Kerr was speaking at a news conference in Belfast on Monday.
He said the decision to hold the conference and appeal to the public for help was an indication of how concerned police are.
It is understood that dissidents are planning to attack what they call "commercial targets".
"There is no point in trying to apply logic to the actions of these groups. They remain unrepresentative remnants from a past that no-one wants to return to," ACC Kerr said.
"But they are a dangerous remnant and they retain the intent and capability to do real and serious harm to communities across Northern Ireland, including their own."
ACC Kerr appealed to people to be vigilant in spotting any suspicious objects when out shopping; to be patient as police officers worked to secure areas and to pass on information to the police.
Last year, dissident republicans carried out a number of attacks in Belfast in the run-up to Christmas.
A small bomb exploded in the city's busy Cathedral Quarter district, and in a separate incident, a man set himself alight inside a shop when the device he was carrying seemingly ignited prematurely inside his jacket.
There was an attempted car bomb attack close to the entrance of Victoria Square shopping centre.
Gunmen also opened fire on a passing police patrol in the Crumlin Road in the north of the city.