Two Catholic churches have been daubed with anti-abortion graffiti, one of which accused Sinn Fein of being "Irish baby killers".
The first incident, which police say occurred between 11pm on Sunday night and 8am the next morning, took place at St Patrick's Cathedral in Armagh.
The graffiti, which condemned the potential repeal of the Eighth Amendment in the Republic's upcoming referendum on abortion, stated: "Save the 8th. Save Ireland."
A separate message, apparently referring to Sinn Fein's support for the repeal of the amendment and the allowance of abortion in certain circumstances, added: "Sin (sic) Fein Irish baby killers speak up.'
Another statement said: "The new reformation is here."
Yesterday, a day after the first incident, St Colmcille's Church in Carrickmore, Co Tyrone, was targeted in a similar incident. It is believed to have occurred between 9.40pm on Monday and 8.05am yesterday.
In this instance, the graffiti stated: "Only DUP speaks for Irish unborn to speak Irish and play GAA vote DUP".
Armagh Sinn Fein councillor Garath Keating said he was "absolutely horrified" at the graffiti on St Patrick's Cathedral.
He urged anyone who had an issue with Sinn Fein's political stance or the abortion referendum to "protest at our office or in a public forum, not write it on a church wall".
"There is plenty of opportunity and forums for public discussion in respect of any of the matters, but to take to spreading your message by writing on a place of worship is horrifying and despicable," he told the Belfast Telegraph.
"The serious issue is the lack of respect. People are entitled to their opinion, but there are plenty of public forums to engage in discussion.
"I can't comprehend how anybody could think this is a useful way to convey their point of view.
"Our policy is well-known in regard to the referendum, and it has been available for public discussion for a number of months.
"It's a highly emotive issue, but to engage in that manner isn't helpful to the debate. I don't think there is any room at the table for that sort of behaviour.
"If people have an issue with the referendum or our party's political stance they can come and protest at our office or in a public forum, not write on a church wall, which is mindless."
Cllr Keating issued an "open invitation" for those behind the graffiti to "discuss the issue" with him. He said that the fact that the graffiti was painted on a Catholic Church, an institution which has called for the Eighth Amendment to be preserved, was "beyond comprehension or rationality".
In a statement last month, the Irish Catholic Bishops' Conference condemned the potential repeal of the Eighth Amendment, saying it "would leave unborn children at the mercy of whatever permissive abortion laws might be introduced in Ireland in the future". The attack on the church in Carrickmore was also strongly condemned.
Orfhlaith Begley, Sinn Fein's candidate in the upcoming West Tyrone by-election, called the incident a "sectarian hate crime," and said that those responsible "should be brought to book".
"This disgraceful graffiti attack on Carrickmore Chapel is the second such attack on a church property in 24 hours," she stated. "I have visited Carrickmore Chapel, I have spoken to Fr (Sean) O'Neill (the local parish priest) and parishioners and people are incensed by this blatant sectarian vandalism.
"Anyone with any information on this hate crime should bring it forward to the PSNI."
Police are believed to be investigating whether there is a link between the two incidents.
A PSNI spokesperson confirmed that they had received reports of criminal damage for each incident. She added that "enquiries are ongoing".