The prime minster has paid tribute to the "professional dedication" of the armed forces and police following the terrorist attacks in Northern Ireland.
Gordon Brown praised the "defiance and determination to stand up to criminal violence".
And he told the Commons that the peace marches in Northern Ireland demonstrate the "unity against violence" felt by the public.
Brown said this unity should send a message that the peace process in Northern Ireland "is here to stay".
David Cameron echoed the prime minister's comments, condemning the attacks as the acts of "callous killers capable of shooting people in cold blood".
The Conservative leader called on all political parties to work with the police to help ensure that the criminals are brought to justice.
And he added that Northern Ireland is "not on the brink or staring into an abyss".
The prime minister replied that there was agreement on all sides on the need to "hunt down these criminals".
He condemned the admissions of responsibility for the murders as "sickening".
And he said that the description of civilians as "collaborators" was "absolutely despicable".
For the Liberal Democrats, Nick Clegg said the murders must not be allowed to divide the people of Northern Ireland.
The DUP's David Simpson said that after the recent violence, Northern Ireland "must move forward". He attacked the killers as "vermin" and said they should be arrested and prosecuted. The police should be supported with whatever resources they need, he added.
Responding, the prime minister said he had agreed to support the Police Service of Northern Ireland with "the resources necessary". He told the Commons that the government would do "everything in our power" to bring the perpetrators to justice.
And SDLP leader Mark Durkan said that the young people of Northern Ireland should reject terrorist groups, and added that the latest violence should not set back policing.
The prime minister welcomed commitments from all the main party leaders in Northern Ireland that they are "determined to root out violence".
Brown added that those who gave their lives "are true patriots".
Former Ulster secretary Peter Hain said it is "especially encouraging" that the leaders of republicanism have condemned the atrocity in Northern Ireland.
The prime minister confirmed that both the first minister and the deputy first minister in Northern Ireland had agreed that "violence cannot be tolerated".
He said that both men had condemned "absolutely" the killing of members of the armed forces and the police.
"Out of this tragedy something is happening that shows that the people of Northern Ireland as well as the politicians want something to happen," Brown added.
Dods Parliamentary Communications Ltd