Leaders of the Screen Actors Guild publicly declared war on a fellow actors’ union at a Monday rally here, increasing the likelihood of new labor strife in an entertainment industry still recovering from a writers’ strike that ended just four months ago.

Standing outside the union’s Wilshire Boulevard headquarters, Alan Rosenberg, president of the 120,000-member guild, urged supporters to reject a tentative deal reached last month between the American Federation of Television and Radio Artists and Hollywood’s production companies.

“We are engaged in the battle of our lives,” Mr. Rosenberg told a group that appeared to number several hundred and included the actors Ed Asner, Keith Carradine, Justine Bateman and Joely Fisher. “It is essential,” he added, “that we vote down that Aftra deal.”

The crowd responded to the speeches with chants of “Vote No!” and bristled with signs opposing the federation’s contract.

About 40,000 of the federation’s approximately 70,000 members also belong to the actors’ guild. And Aftra’s Los Angeles office resides in the same building as SAG’s headquarters.

The federation’s deal with the Alliance of Motion Picture and Television Producers, which covers about a dozen television series, including “Cashmere Mafia” on ABC and “Curb Your Enthusiasm” on HBO, was approved by that union’s board Friday and is set for a membership vote in coming weeks.

SAG leaders, with their own contract set to expire June 30, have accused Aftra of undercutting their efforts for better terms in hotly disputed areas like new media compensation, the mechanism by which actors grant permission to reuse clips from shows in which they appear, and the integration of commercial products in television shows.

Mr. Rosenberg and the guild’s national executive director, Doug Allen, in a letter dated Thursday, asked Aftra’s leaders to delay ratification of their contract because it was “interfering with SAG’s ability to exercise its leverage for the benefit of all actors.”

Roberta Reardon, Aftra’s president, and Kim Roberts Hedgpeth, its national executive director, responded on Friday with a letter warning that the federation would view any steps by SAG to interfere with ratification of the deal, reached May 28, as “a violation of both the law and the A.F.L.-C.I.O. constitution.”

In a phone interview Monday, Ms. Reardon said she expected to ask the A.F.L-C.I.O. to curtail the guild’s campaign. She added that her union’s lawyers were reviewing possible legal action.

“It’s just unconscionable that one union should start a war against another,” Ms. Reardon said. “This simply demonstrates their ineptitude.”

In an unusually difficult cycle of contract talks, unions representing Hollywood’s directors and writers have both reached new deals. But the Writers Guild of America East and the Writers Guild of America West settled only after staging a three-month strike that began last November, and eventually shut down virtually all television dramas, comedies and late-night talk shows.

In a report issued last week, the Milken Institute, a nonprofit that studies economic and social conditions in California, among other things, said the strike had cost about $2.1 billion in lost output and helped tip the state into recession. Other studies, however, have claimed the economic damage was not as high.

Echoes of the writers’ strike reverberated through Monday’s rally, at which dozens of red-shirted writers’ union members and staff members joined blue-clad actors’ guild supporters. “Unaccustomed as I am to speaking to crowds with picket signs, I’ll have to get used to it,” Patric M. Verrone, president of the West Coast writers guild, joked in addressing the crowd.

Many SAG members joined writers during their strike. And writers returned the favor on Monday, resurrecting strike signs that were papered over with a new message: “WGA loves SAG.”

Still, Monday’s demonstration was relatively small compared with the mass pickets that began the writers’ strike, and fell noticeably short of the demonstrations that accompanied a strike by both SAG and Aftra against commercials producers in 2000.

The two unions, which previously bargained jointly, went their separate ways this year. Aftra’s leaders chose to pattern many points in its new deal on new contracts with the writers and directors, while SAG’s leaders have been striving to better those agreements.

The producers’ alliance declined through a spokesman to comment on Monday’s rally. Negotiators for the alliance and the guild resumed talks at an afternoon session.

At the rally, SAG’s leaders were careful to avoid any specific call for a strike, and at least twice told the crowd that a vote against Aftra’s new contract would simply reopen bargaining.

Uncertainty over SAG’s intentions has already assured what many have called a “de facto strike” beginning in late June, because studios have avoided scheduling movie shoots that would be disrupted by a guild decision to walk out. The guild has not conducted a strike authorization vote, and could instruct members to keep working under terms of its old contract even if a new deal is not reached by month’s end.

On Monday Mr. Allen called rejection of Aftra’s pact a way to send a signal to the producers’ alliance. “We are not done yet! We are not done yet!” Mr. Allen said.