Police in Northern Ireland say they have prevented several attempted murders by dissident republicans since a bomb attack on a prison officer.
They have again warned that they expect dissidents to attempt to carry out more attacks during the next few weeks.
Last Friday, a prison officer was injured in a bomb under a van.
A senior PSNI officer says they are deeply concerned by the numbers involved in dissident organisations and their increasing capabilities.
Assistant Chief Constable Will Kerr said the police are preventing or disrupting three or four planned or attempted attacks for every one that takes place.
He said there are "several hundred active dissident republicans" who are led by a small group of people with "significant terrorist experience". Those individuals were previously members of the Provisional IRA.
ACC Kerr reinforced a warning from another senior PSNI officer last week that they are expecting dissidents to attempt to escalate their activities in the run-up to the centenary of the 1916 Easter Rising.
"We have seen an upsurge in dissident republican activity over the last number of weeks and we anticipate that that upsurge might continue," he said.
The PSNI say they have what they call a "high rate of attrition" against dissidents, in terms of arrests, charges and the disruption of activities.
But they are concerned that dissident organisations have been able to maintain their numbers and capabilities, and have appealed to the public to help combat the threat they pose.
"There are several hundred people who can pose a significant threat to the security, to the safety, to the economic prosperity and development of Northern Ireland, and we need community support to stop these groups, to start giving us more information so that we can place more people before the courts, that's what we want to do," he said.
"We know communities have more information about these groupings, who they are, what they do, where they are storing the material, what their plans are.
"The best way we can combat these groups is for those communities to stop viewing the passing of information to the police service as, in some way a bad thing, a dirty word, informing or touting, and see it as a civic duty."
But ACC Kerr said it was important to keep the level of threat posed by dissident republican groups in perspective.
He said their capabilities and the pace of their activity is not on the same scale as that of the Provisional IRA during the Troubles, and will not reach that level "because they have very little public support".
"In some respects they are a bit like a playground bully," he said.
"Everybody else in the school around them has moved on, these people just don't understand what's going on in the playground around them, but the only thing they know, the only thing they can default to is violence.
"Well that doesn't work, it didn't work in the past, it won't work now and it won't work in the future."
This is the second warning from a senior police officer in a week.
Last Friday, Assistant Chief Constable Stephen Martin said the PSNI were "deeply concerned" about the level of threat against its officers, prison officers and soldiers.