Supporters of same-sex marriage have introduced a "palpable climate of fear" into the referendum campaign, according to former senator Des Hanafin.
Mr Hanafin said many No voters have been unfairly portrayed as "homophobic bigots" and have been subjected to a level of intimidation that should have no place in a democracy.
The former Fianna Fáil politician, whose daughter is Mary Hanafin, has been involved in a number of referendum campaigns in the past.
But he said the marriage equality campaign has seen an unprecedented level of fear and intimidation.
"I can recall no election or referendum campaign previously in which so many people felt afraid to say what they truly believe. There are reasonable concerns to be raised about the Government's proposal, but they are being treated as if they are not," Mr Hanafin said.
In Carrick-on-Shannon, people erecting No posters were "run out of the town" by those on the Yes side.
And he said every single No poster was pulled down within a day in his own hometown of Thurles.
Mr Hanafin also criticised public servants, including gardaí, who have called for a Yes vote.