As Ireland gears up for a historic referendum on abortion scheduled for May 25, there are growing concerns that campaigners are resorting to some of the same tactics used in the U.S. presidential election and Brexit referendum to sway undecided voters.

Social media have been awash with bots retweeting opinions and articles with a strong anti-abortion-rights message, according to observers, while Facebook users have been bombarded with privately sponsored or anonymous ads that are popping up in their feeds in an attempt to influence their decisions, according to data from the nonprofit Transparent Referendum Initiative. [1]

On Wednesday, Google Inc. GOOG, -0.40%[2] said it will block all ads about the referendum from its search engine and YouTube.

“There is no doubt there’s an incredibly well-funded opposition campaign,” said Tara Flynn, an Irish comedian, actress and activist for abortion rights. “They were ready to go from the start, and they raised a quarter of a million [euros] in one day. There’s a great deal of misinformation in the public domain.”

David Quinn, founder of the socially conservative Catholic group the Iona Institute, dismissed those concerns: “Of course mainstream media dislike social-media advertising by pro-lifers,” he wrote in a Wednesday tweet. ”Such ads are a way of doing an end-run around media bias.”

Matthew Mulligan, a journalist at the social-media intelligence and news agency Storyful, which sources and verifies online content, said his company has found many videos and digital ads from anonymous sources that are fake. (Storyful is owned by News Corp, the owner of MarketWatch parent Dow Jones.)

In one example, a video purporting to represent the official “no” campaign, featured a man posing as a hospital porter who claimed he had worked in England in “abortion wards.” Another video was uploaded by hackers to a fake YouTube account [3]called RTÉ News, after Ireland’s national broadcaster RTÉ and included people posing as doctors, discussing their regrets about their past involvement with abortion.

“There are Facebook FB, +0.47%[4] accounts like Undecided on the 8th or Facts About the 8th, that are supposed to be neutral, but often they are not linked, there’s no website address, so it’s a bit of a Wild West,” said Mulligan. “The problem is there’s no regulation. Anyone can make an ad. There’s no official registration required.”

Man tearing repeal signs down on Talbot Street just now. Shouts "Vote yes is a vote for murder!" Wearing a blue jacket with an Irish harp on left breast #repealthe8th #savethe8th pic.twitter.com/dm9ecgEtE3[5][6][7]...

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