Ai Weiwei lives his life in public: blogging his anger at the Chinese government, transforming his detention into harrowing dioramas, and now Instagramming up a storm from his exile in Berlin. Over the last two years, the world’s most famous artist-activist has been traveling to refugee camps from Greece to Iraq and Gaza to Myanmar, documenting the displacement of millions and the borders they are desperate to cross. Others might have stayed behind the camera. Mr. Ai, now a refugee himself, puts himself right in front.

The worldwide refugee crisis is the subject of “Human Flow,” Mr. Ai’s new film, and it also informs a gargantuan undertaking of new public artworks in New York, running from Harlem to Flushing and united under the title “Good Fences Make Good Neighbors.” Put aside its cheesy title, cribbed from Robert Frost, and there is a fair deal to admire in this new endeavor, which consists of new sculptures in the form of steel barriers; hundreds of lamppost banners of refugees past and present; and interventions at bus stops across the five boroughs.

Remember the 1990s, when the hawkers of globalization told us this century would see borders fall? In fact construction of international walls and fences has surged worldwide to deter unauthorized migration; Europe’s borderless Schengen zone is under existential threat; Brexit promises to divide Ireland once again; and, if someone has his way, a “big, beautiful wall” may rise south of here soon. Mr. Ai saw those barriers firsthand while filming “Human Flow.” Now he has brought them to New York, where they fit in with alarming naturalness.

This week I saw a solid hunk of the hundreds of small and large additions Mr. Ai has made to New York’s streets and parks. Out in Queens, Mr. Ai has encircled the Unisphere — the stainless steel globe that’s the primary symbol of the 1964 World’s Fair — with a running mesh lattice that rises to about knee height. “Circle Fence” cannot be traversed; this is an insuperable border. Yet the nets’ soft and pliant forms, which you’re free to touch or sit upon, may put you in mind of fishermen or trapeze artists more than of guards and wardens.

There’s a similar tension between menace and shelter in a series of fences and barriers erected in the East Village and in Harlem. The facade of Cooper Union’s original building — where Abraham Lincoln gave the address that drove him to the presidency in 1860 — has been retrofitted with five chain-link baffles that turn its north portico into a prison yard. The burlesque nightclub The Box, over on Chrystie Street, is now topped by a double-height metal grille, as if for riot prevention. And commuters along 125th Street will see metal barricades behind two bus stops, gently curving behind their rear glass panels. Similar barriers have been erected elsewhere in Harlem, and behind bus stops in Brooklyn and the Bronx.

The strongest of Mr. Ai’s new sculptures is “Gilded Cage,” standing 24 feet tall at the southeast entrance to Central Park. This elegant, quietly ominous pavilion consists of an inner ring, inaccessible to viewers, fenced off by hundreds of soaring arched steel struts. A small section of the inner ring has been cut out, so you can walk into the heart of this threatening pergola. Look up from inside, and Mr. Ai’s sculpture resolves into abstract beauty; look into the central ring, and you’ll see its symmetry disrupted by turnstiles familiar from the New York subway, or United States-Mexico border crossings. (You may remember that Mr. Ai once had another set of sculptures on view in this square: his “Zodiac Heads,” which appeared during his detention in 2011.)

In many of Mr. Ai’s best sculptures, repeated forms are freighted with historical or political overtones and yet remain ice-cold — whether in “Template” (2007), his collapsed assemblage of 1,001 Ming and Qing Dynasty doors, or “Straight” (2008–2012), his harrowing arrangement of steel rods recovered from buildings destroyed in the Sichuan earthquake. “Gilded Cage” continues that idiom, even as it relies on prefab metal instead of found materials. As in the sculpture of Mona Hatoum or Rachel Whiteread, this excellent new work uses Minimalism to deliver a very un-Minimal emotional jolt. That extends to the monochrome paint job — a buff gold that echoes Augustus Saint-Gaudens’s nearby memorial of William Tecumseh Sherman, as well as the fastidiously polished brass in the atrium of the nearby Trump Tower.

The counterpart to “Gilded Cage” is the even taller “Arch,” which occupies nearly the whole space underneath the marble arch in Washington Square Park. This simpler, unpainted steel cage is pierced by a mirrored opening, its form suggestive of two conjoined figures. They may appear to bystanders as weary travelers, though mega-fans of Marcel Duchamp will pick out the reference: The figures quote the French-American artist’s 1937 design for the entrance of André Breton’s Paris art gallery. Mr. Ai’s invocation of the Master in this location has a sideways political salience, if you know your downtown history. During World War I, Duchamp and his buddies broke into the Washington Square Arch and proclaimed an “independent” Greenwich Village republic, not subject to the laws and borders of the world outside.

Compared to the sphinxlike “Gilded Cage,” “Arch” wears its convictions more publicly. This is a big, public ode to freedom of movement, yoking America’s first president (a dissident) and most influential Dadaist (an immigrant). The test of a work of art’s success, though, is not how fluently it communicates a single message; the test is how forcefully it reflects, unsettles, and transforms the world in which it intercedes. By that standard, “Gilded Cage” stands as the greater achievement, enfolding inside and outside, warden and captive, into a single, synthesized public form. “Arch,” by contrast, offers less, and risks being remembered only as a selfie backdrop for woke narcissists.

Bridging all these works are two hundred lamppost banners, depicting immigrants and refugees — some of whom Mr. Ai photographed in Iraq’s Shariya refugee camp, others snapped on his cellphone during his travels for “Human Flow,” and still more borrowed from historical sources. (Among the last category: Emma Goldman, floating above 7th Street.) Rather than printing the images with ink, the artist used a laser cutter to remove the white space from each photograph; each banner, therefore, is a cutout negative of a refugee, and the sky and the city are visible through their faces.

If you’ve seen the Guggenheim’s muscular, rigorous new exhibition, “Art and China After 1989,” for which Mr. Ai has curated the film program, you will not be surprised by the forthright advocacy of these lamppost portraits, as well as “Arch” and other blunt interventions at city bus stops. In the 1990s he took an impish tack, whether he was photographing a flasher in Tiananmen Square or dropping a millennia-old Chinese urn, in biting parody of both western performance art and Cultural Revolution iconoclasm. His art turned to direct advocacy in 2008, when he began his essential “Citizen’s Investigation” of the death toll of the 2008 Sichuan earthquake, whose results are on view at the Guggenheim. Since then he has reoriented his sculpture, videos, and social media accounts to serve almost as a broadcast medium for Chinese and global freedom — and, as a result, he has endured frequent gripes that his activism has got the better of his art.

I’ve always found that gripe to be unfounded. Like his hero Duchamp, Mr. Ai has wholly erased any border between his art and his life — and there are some emergencies, among them the displacement of more human beings than any time since World War II, that this artist can only address with bluntness. That sometimes lends itself to less challenging sculpture, like in Washington Square, or simple boosterism, as in the refugee portrait banners. Step back, though, and look at the project in aggregate, and “Good Fences Make Good Neighbors” displays all the confidence and moral passion of his most important later projects.

One of the great surprises of this citywide artistic outcry is that Mr. Ai’s obstructions — “very almost-art, but maybe, maybe not,” as he told The New York Times last week — don’t actually disrupt the city very much, but plug into the urban fabric of New York with an ease I found disturbing.

Passengers waiting for the bus on 125th Street behind Mr. Ai’s barricades went right on with their commutes. Tourists in Corona Park were taking their selfies with a fence in frame. At Cooper Union and in Washington Square, metal barriers from the N.Y.P.D. echoed the artist’s own. South of “Gilded Cage,” shoppers on Fifth Avenue wended through ad hoc concrete obstacles around the president’s own tower. Mr. Ai’s citywide checkpoints are a hundred muted bells that add up to a deafening alarm: We have accepted so many physical and political limits that new ones go unnoticed, and we may not protest our shrinking freedom until it’s too late.