These aren’t just Twitter hashtags. They’re also the slogans that adorned posters at protests across the country Saturday, as tens of thousands rallied in Washington, New York, Boston, Los Angeles and elsewhere to call for changes to the American criminal justice system.

The phrases were borrowed from the conversations about race and law-enforcement violence that have unfolded online in the months since this past summer, when Eric Garner died at the hands of New York police and Michael Brown was shot to death in Ferguson, Mo. After those deaths, activists launched hashtags that drew attention to their cause and acted as a kind of digital trademark for the content of their tweets.

All of that makes sense, says Philip Howard, the principal investigator at the Digital Activism Research Project at the University of Washington in Seattle. In the past few years, a solid Twitter presence has practically become a prerequisite for any good protest movement.

What’s unconventional is that participants in Saturday’s marches took those hashtags and wrote them on posterboard. That one of the Internet’s most dynamic phenomena is appearing in a format that can’t be clicked or linked to or collected as data says a lot about the relationship between what happens online and “in real life.”

Namely, that the Internet is real life.

“I don’t think it makes sense to distinguish online activism from offline activism anymore,” Howard said.

Instead, he sees digital and traditional activism as feeding into one another. News of the events in Ferguson surfaced on Twitter early on, in many cases before the media picked up on it. Such hashtags as #BlackLivesMatter and #IfTheyGunnedMeDown and #CrimingWhileWhite helped stoke online conversations about race and the criminal justice system.

Meanwhile, Saturday’s posters performed the dual role of carrying those conversations off-screen and marking the day’s events as something that should be posted online once the protests were over.

“The information that comes out over social media fuels the protest itself . . . and the content of what happens flows back into the online realm,” said Deen Freelon, an assistant professor at American University who studies digital activism and who attended Saturday’s “Justice for All” march in Washington.

This interplay between real-world events and online activism is what turns a catchy hashtag into a powerful slogan.

“It tethers the conversation to things that really happened,” Howard said.

Howard, who calls himself a “cyber-optimist,” acknowledges that his argument isn’t conventional wisdom. Hashtag activism — which includes such movements as #Kony2012 and #BringBackOurGirls — often draws criticism, with detractors characterizing so-called “slacktivism” as a lazy, passive form of protest; they emphasize that tagging a tweet with #BlackLivesMatter or #OccupyWallStreet doesn’t equate with the effort it takes to attend a protest or lobby Congress.

But activists, including organizers of Saturday’s protests, said that the online conversation about the deaths of Brown, Garner and other black males who have been killed by police recently has helped their cause.

“The hashtag function is so that you can find everything that’s under this category, but it carries with it this idea that we’re all on the same page — we’re all talking about the same thing,” said Sabaah Jordan, an organizer of the “Millions March.

A pervasive hashtag such as #BlackLivesMatter helps connect the digital dots between incidents. Created in 2012 in response to the death of Trayvon Martin, its use surged after grand jury decisions not to indict the officers involved in the deaths of Brown and Garner.

As communications coordinator for the New York march, Jordan used the #BlackLivesMatter hashtag to promote the event.

“For getting attendance, it’s like having access to a billboard for free,” she said.

She wasn’t surprised to see the phrase turn up on posters; it seemed like an appropriate, succinct summation of what the march was about, she said, as well as a nod to the role that Twitter played in organizing it.

Howard does note that the expanding role of digital activism in protest movements carries drawbacks. Chief among them: Hashtags are much better at uniting disparate groups around a single issue than they are at creating a lasting political movement.

Just look at Middle East, Howard says, where protesters were early adopters of Internet-to-IRL activism, including techno-jargon posters. Howard recalls seeing a sign from Egypt’s Tahrir Square during the Arab Spring protests that read, “Control + Alt + Delete Mubarak.”) But in the nearly four years since that uprising, the activists who launched those protests haven’t been able to form a cohesive party or exert lasting political power.

“Having a powerful hashtag means capturing public attention for a few days or making a story stay in the headlines a little bit longer than it normally world,” Howard said. “But these things can be fleeting.”

Will appearing on posters and at protests make hashtags less ephemeral? Freelon doesn’t think so, though he’s won’t dismiss them entirely, either.

“Social media is not a substitute for activism or protests — it’s a complement to it,” he said. “We’re just still trying to figure out how.”