Sinn Féin organised the protest at PSNI Headquarters in Knock after Pádraic Wilson was remanded in custody on Friday for alleged IRA membership, a charge they say is "politically motivated".
Belfast Magistrates' Court heard that Wilson had talked with the sisters of Robert McCartney after he was killed outside a bar in Belfast city centre.
Statements from the sisters alleged he met with them at least twice, and he had claimed to be a member of the Provisional IRA's senior council.
Wilson, 53, of Hamill Park, Andersonstown, Belfast, denies charges of IRA membership and two counts of addressing a meeting to encourage support for the outlawed grouping.
All of the alleged offences were committed within two months of Mr McCartney being stabbed to death outside Magennis's bar in the city centre in January 2005.
The 33-year-old father of two, from the Short Strand in east Belfast, is believed to have been in an argument with republicans before he was knifed.
Although no one has been convicted of Mr McCartney's murder, his five sisters and his partner waged a major campaign to bring those responsible to justice.
Sinn Féin's Gerry Kelly has demanded the immediate release of Wilson, labelling the charges against him "bizarre".
We believe that this arrest is politically motivated, that it comes from within the Serious Crime department of the PSNI and it is, frankly, a small minority still within the PSNI who are against new beginnings to policing and who are using things like this to put it into crisis.
Gerry Kelly, SF
Mr Kelly said the allegations were driven by an "old agenda within the PSNI". He said he would be raising the issue with the Policing Board.
"They aren't trying to find answers to long-running questions," he said.
He added: "This is not about bringing the killers of Robert McCartney to justice, it's quite the opposite. It is targeting people who wish to help.
"Police have failed to bring forward someone for the killing of Robert McCartney to justice."
During a press conference with fellow party members Catriona Ruane and Alex Maskey, the north Belfast MLA called on the Chief Constable Matt Baggott to review what was happening within the Serious Crime Unit.
"Throughout the mid 90s and up until his release in 2000, Pádraic Wilson played a pivotal role in building support among prisoners for the peace process," said Mr Kelly.
"Since his release he has continued to play a central role in supporting and developing the political process. His arrest represents a very serious challenge to the political process."
SDLP MLA Alex Attwood has questioned Sinn Féin's response.
He said: "All who value justice should stand full square with the McCartney family and others, fearless in their determination that the guilty account for their actions.
Mr Attwood said Mr Kelly's comments were a "clear attempt to mislead".
"In previous years, Sinn Féin complained of 'political influence' over the courts and of 'political policing'.
"Today, in a brazen public way, Sinn Féin are at the same. People campaigned, worked and struggled to change 'the old order of things', for now Sinn Féin to recreate it, a modern version of the failed past, a renewed attempt to pressure politically the due process of law.
Sinn Féin's position is, on one hand, to call on people to provide information to the police but, on the other hand, not to do so when it relates to a Sinn Féin member. This is how shallow Sinn Féin's position is and no-one should buy into it.
Alex Attwood, SDLP
DUP MLA Maurice Morrow has said the party's reaction was "Sinn Féin reverting to form".
He said: "I repeat my earlier call for Wilson to be stood down by Sinn Féin as and until the case against him is concluded.
"Mr Kelly must accept due process must take its course and should not be interfered with politically."
Wilson, a former IRA leader in the Maze Prison, was arrested and charged by detectives from the PSNI's Serious Crime Branch.
Opposing his application for bail on Friday, a detective from the PSNI's Serious Crime Branch claimed there was a risk of re-offending and interference with witnesses.
He was remanded in custody to appear again by video-link later this month.
The PSNI have not responded to Mr Kelly's comments due to the ongoing court case.