Twenty-nine people were killed at Omagh
The families are calling for the extension of anti-terrorism legislation to cover those funding the Real IRA.
The legislation is currently going through Parliament.
Twenty-nine people died and more than 200 were injured on 15 August 1998 when the Real IRA bombers left a massive car bomb in Omagh town centre.
The emergency Anti-Terrorism, Crime and Security Bill is aimed at international groups and was produced in response to the attacks on the United States on 11 September.
The Omagh families have said the legislation should ban the 32-County Sovereignty Committee and the Irish Prisoners' Welfare Association, both of which have, in the past, been linked to dissident republican elements.
'Universal law'
Michael Gallagher, whose 21-year-old son Adrian died in the Omagh bomb, said: "These people are continuing to organise and fund-raise both in Ireland and Britain.
"The government has the opportunity to do something about it and yet they are being selective on how they are dealing with terrorism.
"The government should have made the law universal - it could cover all organisations, because a terrorist is a terrorist at the end of the day."
He said the laws should apply to those people bombing Britain and Ireland and urged the law to be changed to close loopholes permitting "front organisations" for terrorists.
"I think people have to realise that these people use the democratic system and eat it from within, and the government seems powerless to do anything about that," he said.
The Omagh relatives have asked for a meeting with Prime Minister Tony Blair and Home Secretary David Blunkett in an effort to have the legislation extended.
On the third anniversary of the atrocity in August, senior policemen from both sides of the Irish border said they would not rest until they brought the Real IRA bombers to justice.