As City Council business goes, changes in the building code are often considered sleepy agenda items. But hundreds packed City Hall chambers yesterday for a long, contentious hearing focused on overhauling the city’s antiquated building code, a task thrust to the forefront after the World Trade Center buildings collapsed on September 11, 2001.

On one side of yesterday’s hearing were plumbers, steamfitters, firefighters, and other union members who favor a template designed by the National Fire Protection Association. They said it requires more stringency with respect to safety measures.

On the other side were representatives of associations of builders and engineers, who advocated revisions modeled on the International Building Code. That’s the version the Bloomberg administration backs as well.

Both models would have to be amended for New York City.

Two competing bills in the council, supporting the different versions, crystallize just how divisive the issue has become. At yesterday’s hearing, the hundreds of union workers held signs expressing support for Intro. 368, while building lobbyists wore buttons that read Intro. 478.

No side-by-side comparison is available to show exactly how the codes differ, because the models are essentially just that: models, not outlines. Some said builders support the international model because compliance would be less expensive, and that the unions prefer the national model in part because it could generate additional work.

A mayoral commission appointed in 2002 recommended last year that the city adopt the international proposal and amend it to accommodate special city requirements.

But Council Member Robert Jackson, the lead sponsor on the rival bill, called the commission a “setup” and said the administration began pushing the IBC model before it began studying the issue.

The Department of Buildings has been comparing the city’s current code with the international document for the last year and would have to start the process over if it were directed to implement the other model.

“In my opinion, what you’ve done is you put the cart before the horse,” Mr. Jackson, Democrat of Manhattan, told the department commissioner, Patricia Lancaster.

She denied that the commission started the process with a conclusion in mind, saying the panel assessed both models in 10 categories and concluded the international was “superior” across the board.

The representatives of several other agencies who are on the commission, including the chief of the Fire Department, Peter Hayden, also testified in favor of the international model. Mr. Hayden said it would be “terribly misguided” to approve the national model.

In addition, Ms. Lancaster said it would be easier to implement the international code because it was already used in 44 states and had recently been adopted by New York State. She also argued it would stimulate growth of affordable housing and spur economic development, while maintaining safety standards.