Architects and preservationists including Robert AM Stern were among those protesting Snøhetta's plans for Philip Johnson's postmodern AT&T Building in New York today.

A small crowd attended a planned protest at the base of the Midtown Manhattan skyscraper this afternoon, carrying signs that read "Hands off my Johnson", "Granite is great" and "Save AT&T". Reporters from Metropolis magazine captured images of the protest, as did several Instagram users.

Among the pack was New York architect Stern, carrying a model of the building – replicating a 1979 Time Magazine cover that shows architect Johnson doing the same – and British architect and postmodernism enthusiast Adam Nathaniel Furman. It was organised by filmmaker Nathan Eddy, who also launched a petition against the project.

The group was rallying against a proposal to replace the iconic skyscraper's base with a scalloped glass frontage, unveiled by Snøhetta earlier this week.

The design triggered outcry from several members of the architecture community, with two online petitions – Eddy's and another created by Swiss journal Archithese – launched shortly after it was revealed.

British architect Norman Foster promoted the protest on his Instagram account, but was unable to attend himself.

"I was never sympathetic to the short lived postmodern movement – and this building in particular," he wrote in the caption. "However it is an important part of our heritage and should be respected as such."

Critics Alice Rawsthorn, Alan G Brake and Alexandra Lange were also among those to post, like or retweet about the petitions and protest on Twitter, using #saveatt.

Dezeen spoke to members of the architecture community to gauge their opinions on the project.

Guardian architecture and design critic Olly Wainwright, who described the plans as "vandalism" in an earlier tweet, told Dezeen that altering the tower's base was akin to adding code-compliant steps to the sloped roof of Snøhetta's widely praised Oslo Opera House.

"The tower's jaunty Chippendale top might be its most well-known feature, but how it lands on Madison Avenue with a marching row of muscular stone columns, either side of its soaring arched entranceway, remains the defining experience from the street," he said.

"It is an experience that Snøhetta appear determined to destroy in the name of creating a 'lively and identifiable public face' to the building, by slicing it off at the knees and leaving it teetering on a row of matchstick columns."

Wainwright also drew similarities between the project and Snøhetta's recently completed extension to the Mario Botta-designed San Francisco Museum of Modern Art – another icon of postmodernism in the US.

"They claim to be 'breathing new life' into both of these postmodern classics; instead, they are smothering them with their suffocating blanket of Scandi good taste," Wainwright added.

New York architect Richard Meier also expressed his opposition to the AT&T redesign: "From the three images that we see published of the renovation of the lower levels of the Philip Johnson AT&T building, I don't think that the proposed design is appropriate or respectful to the integrity of the original design of the building."

Dezeen contacted Snøhetta for comment is yet to receive a response.

The 647-foot (197-metre) AT&T Building was completed by Johnson and partner John Burgee in 1984 as the headquarters for the telecommunications company. It is considered the world's first skyscraper built in the postmodern style – a movement that emerged in the late 1970s as an ideological reaction against the utopian ideals of modernism.

The building was later taken over by electronics giant Sony and renamed accordingly, but has sat vacant since the company left a year and a half ago. Snøhetta's renovation aims to lure new tenants to the building, which now goes by 550 Madison.