Gerry Adams has been warned of a "credible" death threat, Sinn Fein said.
The party blamed those opposed to the peace process for the message, which it said was passed by police to Mr Adams' wife while he was not at home.
Adams, 65, has been released from Antrim police station, pending a report being sent to prosecutors, after four days of questioning about the notorious 1972 killing of mother-of-10 Jean McConville and other alleged links with the IRA.
Sinn Fein Stormont Assembly member Raymond McCartney said: "The PSNI (Police Service of Northern Ireland) officer told Gerry Adams' wife Collette that they had information of a credible threat to the life of Gerry, who was not at home at the time.
"Clearly there are elements that are opposed to the peace process and anti-Sinn Fein. We will not allow them to succeed nor will we be deflected from our determination to build the peace process."
A police spokesman said the force did not discuss the security of any individual.
He added: "However, if we receive information that a person's life may be at risk we will inform the relevant persons accordingly. We never ignore anything which may put an individual at risk."
Another senior Sinn Fein member, Bobby Storey, was also warned of death threats, the party said.
Adams has vehemently rejected allegations made by former republican colleagues that he ordered the abduction and killing of Mrs McConville.
Tonight at a European election rally Adams claimed his arrest was a sham.
He said the family of Mrs McConville have the right to seek justice, but he claimed he was looking to the future.
The decision whether to charge him with any offence will be made by the Public Prosecution Service (PPS) at a later date after reviewing evidence presented by police.
Also today, a son of Mrs McConville has said Adams threatened him with a "backlash" if he released the names of those he believed were responsible.
Michael McConville has said his family's fight for justice will go on after the Sinn Fein leader was freed, but has maintained he could be shot if he disclosed the identities of suspects to police.
McConville alleged the "threat" was made around the time a report being drawn up by Northern Ireland's then police ombudsman, Nuala O'Loan, into claims that his mother was an informer was close to being finalised.
Asked by CNN's Christiane Amanpour about Mr McConville's claims, Adams said: "First of all I never said that. I have tried my best...
"I can understand absolutely, given what Republicans have done to their family and their mother, I can understand absolutely why the McConville family feel the way that they feel, so let me say that as a matter of record.
"But I am very, very clear - and they may reject this - I have been trying my best to support and help all of the victims, but I have a particular wish to help the victims of the IRA."
The Sinn Fein president had brokered a series of meetings between him and members of the IRA.
McConville said he used to tell Adams what had happened in the meetings and warned him that he would release the names of those involved if Ms O'Loan's report was disputed.
At that point he said the backlash was mentioned.
McConville said that "could" have meant a backlash against the peace process but said he took it to mean the "backlash from republican people".
Helen McKendry, Mrs McConville's daughter, told Channel 4 News the family was "listening to legal advice".
She added: "I want the people who dragged my mother from her home, tortured her, killed her, buried her, I want them brought in a court of law.
"That's all I want for my mother, justice."
Adams will refocus on election campaigning today as the political fall-out from his release from police custody continues to reverberate around Stormont and beyond.
The republican party is holding a European election rally in Belfast tonight, with a similar event planned in Dublin tomorrow, as Adams resumes the canvassing activities he claims his detention was designed to thwart.
The rapturous welcome Adams received in a west Belfast hotel on his first public appearance was in marked contrast to the angry scenes outside the police station as loyalists protested at the decision to free him.
There was disorder in the loyalist Sandy Row area of Belfast, with petrol bombs and stones thrown, though no one was injured.
The former MP for west Belfast and now representative for Co Louth in the Irish Dail criticised the police's handling of his arrest but moved to dispel any suggestion that Sinn Fein's commitment to policing had wavered in the wake of the affair.
His arrest on Wednesday triggered a bitter political row at Stormont, with Sinn Fein accusing an "anti-peace process rump" within the PSNI of orchestrating the detention with the aim of damaging the party ahead of the European and local government elections.
This was angrily rejected by political rivals, whose fury intensified when senior Sinn Fein figures indicated that their support for the police - a critical plank in the peace process - would be "reviewed" if Adams was charged.
Democratic Unionist Stormont First Minister Peter Robinson denounced those remarks as "bully boy" tactics.
Downing Street confirmed that David Cameron and Irish Taoiseach Enda Kenny spoke yesterday to discuss the situation surrounding Mr Adams' arrest.
Adams questioned the timing of his detention and said police had unnecessarily used "coercive" legislation to detain and quiz him.