Legislation allowing fugitives from Northern Ireland to return home is due to be published.
The proposals cover up to 150 people wanted for crimes committed before the Good Friday Agreement in 1998.
They would have their cases heard by a special tribunal, and if found guilty would be freed on licence without having to go to jail.
The DUP has brought relatives of terrorism victims to Westminster to lobby against the scheme.
Unionist, Conservative and Liberal Democrat politicians are also opposing the proposals.
Between 40 and 150 fugitives could benefit from the scheme, including the former Sinn Fein publicity director Rita O'Hare and the IRA Maze escapee Pol Brennan.
On Monday, Prime Minister Tony Blair defended the government's plans, saying those suspected of offences before 1998 had to be dealt with in the same way as those prisoners released under the Good Friday Agreement.
The tribunal hearing the cases would consist of a retired judge sitting without a jury; the accused would not have to turn up for the hearing, but could send their lawyer in their place.
If found guilty, they would be freed on licence without having to serve any time in prison.
Aileen Quinton, who lost her mother Alberta in the 1987 Enniskillen bombing, said the families wanted justice.
She is in London with other victims' relatives to lobby against the legislation.
"Some things are so important that you just have to do them," she said.
"We are not looking for vengeance, we are not looking for sympathy, we are looking for justice and justice has to be the bedrock of any kind of peaceful or decent society."
On Tuesday, Northern Ireland Secretary Peter Hain said security force members should receive equal treatment.
He said those in the services who found themselves charged with crimes committed before 1998 should not be discriminated against compared to paramilitaries.
In October, Mr Hain said "dozens" of paramilitary fugitives could be allowed to return to the province under the legislation.
He also said there would be "a proper judicial process".
However, Conservative Northern Ireland spokesman David Lidington has been critical of the plan.
"At the same time as the government proposes detaining terrorist suspects for 90 days without charge, it will allow convicted terrorists, and those wanted in connection with the most heinous terrorist atrocities, to return to Northern Ireland without ever having to appear before a court and account for themselves in the normal way," he said.
DUP MP Jeffrey Donaldson said his party had made it clear "that this issue is crucial".
"This could do a lot of damage to public confidence," he said.
"We will not move into devolved government in circumstances that there is not the confidence in the community in the political process," he said.
"We have a government who today on the one hand are proposing strong legislation against terrorism... and on the same day proposing legislation that grants and effective amnesty to some of the most notorious terrorists in the UK."
Sinn Fein MP Martin McGuinness said many people had been on-the-run since internment more than 30 years ago.
He said while many people had suffered pain, the legislation was "a sensible measure".
"We have to see all of this as part of a bigger picture... the peace process has transformed the lives of people in the north.
"What is the sense of people being pursued whenever everybody knows that as a result of the releases under the Good Friday Agreement they are not going to spend one day in prison."
BBC Northern Ireland's political editor Mark Devenport said the government promised to allow paramilitary fugitives to return home back in April 2003, but held back the move until the IRA called off its campaign and disarmed.
"The law is set to be published, and could face a rough ride in parliament, especially in the House of Lords," he said.