Paramilitaries are attempting to destroy the peace process, the Northern Ireland secretary has warned.

Dr John Reid described the murder by dissident republicans of a Derry Territorial Army civilian worker as a "brutal and cowardly act".

Police are not yet sure whether the bomb was the work of the Real or Continuity IRAs.

David Caldwell, who served in the Ulster Defence Regiment, was killed after a booby-trapped lunchbox exploded.

Reid attacked hardliners for trying to derail peace in the province.

"There is a myth going around that this violence is only the result of the peace process, as if it never happened before," he said.

"The reality is that there is a lot less violence than there was 10 years ago but it is escalating at present because those extremes on both sides desperately want to kill the peace process and put themselves back centre stage in dominating the politics by the gun."

He warned dissident paramilitaries that the government would do all it could to crack down on their activities.

"I can't promise the people of Northern Ireland that these people, in their brutal and cowardly fanaticism, will not attempt to or ever get through again," he told the BBC.

"I can promise them I will do everything possible and so will the police service and the security services."

"But the people who really have to show the resolution and the endurance, and to recognise that by giving up the peace process they meet the aims and objectives of the gunmen, are the ordinary people of Northern Ireland themselves who will be taking to the streets today in the rally that is being held".

Tens of thousands of people protesting against sectarian violence were expected at Belfast City Hall later on Friday.

The rally of community leaders, clergy and trade unionists was called following the murder of Catholic teenager Gerard Lawlor in the north of the city last week.

City councillors are supporting the demonstration but hard-line unionists are boycotting the event because of the presence of Sinn Fein's Belfast lord mayor Alex Maskey.

Sin Fein has condemned the latest murder. Mid Ulster MP and Stormont education minister, Martin McGuinness, said the killing was "absolutely and totally wrong".

"These attacks, whether by rejectionist loyalists or dissident republicans, are attacks on the peace process and must be condemned in the most forthright and unequivocal terms," he said.

Democratic Unionist leader the Reverend Ian Paisley, who is boycotting the Belfast peace rally, accused the Nortehrn Ireland secretary of perpetuating his own myths."He should tell that to the families of the two most recent murder victims, David Caldwell and Gerard Lawlor.

"At our meeting with the prime minister in Downing Street earlier this week, at which the secretary of state was present, he was confronted with incontrovertible figures confirming the massive increase in shootings and bombings since the agreement was signed," he said.

"Despite this, he continues to state that the situation is much better, though he has to go back 10 years to make his comparison. Violence today is very, very real and it is not just in the interface areas, as we have seen with the callous murder in Londonderry yesterday."