It's understood the 52-year-old man was on his way to work when the device exploded from below his van on Hillsborough Drive in east Belfast.
He was sent to hospital with serious injuries, where he underwent surgery and is now in a stable condition.
Police Service Northern Ireland (PSNI) has evacuated people from their homes on Hillsborough Drive and have cordoned off the street and several others in the immediate area.
The Service has said there is an "ongoing security alert" after the blast, with other police and prison personnel in danger.
The Temple, a Salvation Army church close to where the incident took place, has opened its doors to evacuees and has taken care of at least seven people so far.
Iris Corry, from The Temple and is also part of Belfast's emergency response committee, told Premier's News Hour: "We're simply doing the work Christ would want us to do, to serve people at the point of need, and show His love in that way.
"We've just made a place here of shelter for people to come, where they'll feel welcome and we can just listen to them and let them know that we're here for them and we're showing God's love.
"Any prayers that people want to offer, I believe that that will give us all strength."
Northern Ireland Secretary Theresa Villiers said: "I utterly condemn this vicious attack on a prisoner officer East Belfast.
"Like all his colleagues in the prison service, this officer serves the whole of the community, in stark contrast to the people who carried out this appalling and violent crime.
"I strongly urge anyone with information about this murder attempt to contact the police to help bring those responsible to justice".
Listen to Premier's Alex Williams speaking to Iris Corry on the News Hour:
