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After outrage quickly spread online, a billboard in Boston from a group denying the Armenian genocide had a remarkably short run this week.

It appeared in the North End Tuesday night. By mid-morning Thursday the billboard’s owner Clear Channel Outdoor peeled it away and replaced it with an Ad Council spot promoting adoption.

It featured the words “Truth = Peace” and a link to the site FactCheckArmenia.com and the statement: “Proudly paid for by the Turkic platform, Istanbul.”

Clear Channel spokesman Jason King later said the ad had been “placed there in error.”

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A Twitter firestorm about the ad could have started anywhere, but it started with Somerville’s Elizabeth Weinbloom.

“Horrific billboard in Boston’s North End, denying the Armenian Genocide. @marty_walsh, do something,” she tweeted around 1p.m. Wednesday.

Her tweets about the ad were retweeted more than 100 times and spurred many others to pepper Mayor Marty Walsh, MassDOT and Clear Channel with calls to action. There was also awidely circulated petition.

Weinbloom, a onetime candidate for Board of Aldermen, said in an interview she couldn’t believe how quickly it all happened.

“I guess I’ve never had the privilege to experience social media being so incredibly effective at correcting an injustice,” she said. “Once they got 200 tweets at them, they backtracked” – referring to the billboard-owning company.

She said she never saw the billboard in person. The picture she shared came from the Facebook page of a friend, North End resident Jeremy Koo. Most of the extensive barrage of tweets that followed happened from her seat on a bus bound for New York, Weinbloom said.

“I was almost concerned that maybe it had never been there,” she said. “I said, ‘You definitely saw that billboard yourself last night, right?’”

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Koo said in an email that he spotted the ad on his way home via Government Center. From a distance, he said, he assumed it was some kind of call for global unity. Realizing it wasn’t was “infuriating,” he said. So he snapped a picture.

Weinbloom was especially sensitive to the issue, she said, and especially tuned into the ad’s coded messaging – or “dog whistles” as she called them. As it happens, she spent time last year traveling in Turkey and studying the Turkish government’s opposition to the genocide designation.

“That is a billboard that is 100 percent meant to be seen and understood by Armenians and Turks exclusively,” she said.

The U.S. is among many countries that do not recognize the killing of hundreds of thousands of Armenians in the early 20thCentury as genocide – although 43 states, including Massachusetts, do.

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The ad was not far from Boston’s Armenian Heritage Park and Holocaust Memorial. And its placement came just after a visit from Armenian president Serzh Sargsyan and before an annualWalk Against Genocidein the area.

This being Twitter, when Weinbloom’s message caught on, she braced for hate-filled responses. There were some, but not many. Some read her last name and fired off anti-Semitic comments or confronted her about Israeli policy. Most sent encouraging words.

Among them, she got this message from a friend: “Only use your powers for good.”