No effort would be spared in "hunting down" those responsible for the murder of Ms Rosemary Nelson, the British Prime Minister has said.
Mr Tony Blair condemned this "disgusting act of barbarity" and said her murderers wanted to remove any chance of reconciliation in Northern Ireland.
But "they will not be allowed", he said. "No effort will be spared in hunting down and bringing to justice those responsible for this senseless and despicable act of murder," he added.
Mr Blair met Ms Nelson last month when she visited Downing Street with a delegation representing the Garvaghy Road Residents' Coalition.
The North's Political Development Minister, Mr Paul Murphy, said: "We will leave no stone unturned in the effort to bring the perpetrators of this evil crime to justice. Such attacks will only strengthen our determination to make the Good Friday agreement work and ensure that violence is a thing of the past." The Conservative leader, Mr William Hague, denounced Ms Nelson's murder, which he said was a "terrible and horrible crime", designed to derail the peace process. He appealed to the people of Northern Ireland to persevere with their efforts to reconcile both traditions. He hoped they would "not be deflected by people who still believe in violence".
Further condemnation came from the Liberal Democrat spokesman on Northern Ireland, Mr Lembit Opik, who said: "This murder should remind us how important it is to ensure that the last remnants of terrorism in Northern Ireland are not allowed to divert us from peace."
Turning to those responsible for Ms Nelson's murder he added: "They will try anything to stir up the old conflict. The only epitaph which will mean anything for those left behind by this tragedy is an end to violence."
The joint presidents of the New Dialogue group - Mr Harry Barnes, Labour MP, and Mr Peter Bottomley, Tory MP - described Mrs Nelson's murderers as "fascists" and appealed to paramilitaries not to retaliate.
In a statement, they said: "Those who carried out this monstrous attack must be hunted down with the utmost rigour by the RUC. Their attack seems to be aimed at undermining the peace process. New Dialogue appeals for no retaliation by anyone and full co-operation with the police in bringing the perpetrators to justice."
Mr Ken Livingstone, Labour MP, has urged the British government to set up an independent inquiry to investigate complaints made by Ms Nelson about death threats made against her by RUC officers.
He said last night that he was planning to table an Early Day Motion in the House of Commons calling on the government to investigate what security measures were taken to protect Ms Nelson's life.
Mr Livingstone said Ms Nelson had reported that she had received no satisfactory response from the RUC about the threats made against her life.
British lawyer Mrs Gareth Peirce, who was a friend of Ms Nelson, said fundamental international law also applied to defence lawyers. Speaking on Channel 4 News, she said: "She ought to have been protected. It would have been known that she was at risk."