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.
Mr 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.
Mr Adams has vehemently rejected allegations made by former republican colleagues that he ordered the abduction and killing of Mrs McConville.
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.
A son of Mrs McConville has said Mr 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.