Three hundred female Hollywood actors, agents, writers, directors, producers and entertainment executives – including Natalie Portman, Emma Stone and Reese Witherspoon – are kicking off the new year with a coordinated effort to counter systemic sexual harassment in the entertainment business and US workplaces.

The initiative, announced with a full-page ad in the New York Times, includes a $13m legal defense fund to help women in less privileged professions protect themselves from sexual misconduct and the consequences that may arise from reporting it.

Entitled Time’s Up, the programme was launched with an open letter signed by hundreds of women in show business, including Ashley Judd, Eva Longoria, America Ferrera, Rashida Jones and Kerry Washington, as well as powerful backroom Hollywood figures, including producer Shonda Rhimes, whose credits include Grey’s Anatomy and Scandal.

“The struggle for women to break in, to rise up the ranks and to simply be heard and acknowledged in male-dominated workplaces must end; time’s up on this impenetrable monopoly,” part of the letter reads.

“If this group of women can’t fight for a model for other women who don’t have as much power and privilege, then who can?” Rhimes told the New York Times.

In a sense, Time’s Up is being launched as a companion to the #MeToo movement that grew out of the spontaneous response to revelations about Hollywood’s “casting-couch” system of sexual predation and enduring gender-pay disparities.

While attention has largely focused on show business and the media, Time’s Up seeks to include the plight of working-class women.

The organization arose from informal gatherings of female talent agents in Los Angeles who starting meeting after the issue of sexual harassment landed like a bombshell on the entertainment industry in October. The group rapidly expanded and now includes meetings and workshops for participants in New York and London.

Organizers said they were moved to broaden the effort after receiving an open letter on behalf of 700,000 female farmworkers in November.

The initiative’s goals also include promoting legislation to penalize companies that tolerate persistent harassment, and to fight against the use of non-disclosure agreements to shield sexual abusers.

Organizers also plan to ask women walking the red carpet at the Golden Globes this year to wear black. “This is a moment of solidarity, not a fashion moment,” actor Eva Longoria, who rose to fame in Desperate Housewives, told the paper.

Janelle Monáe, a signatory of the Time’s Up open letter, at the 2017 Academy Awards. Organizers plan to ask women to wear black on the red carpet at this year’s Golden Globes. Photograph: Ben Peterson/Getty Images

“For years, we’ve sold these awards shows as women, with our gowns and colors and our beautiful faces and our glamour,” Longoria added. “This time the industry can’t expect us to go up and twirl around. That’s not what this moment is about.”

Beyond the symbolism of red carpet fashion, Time’s Up’s legal defense operation will be administered by the National Women’s Law Center’s Legal Network for Gender Equity which connects victims of sexual harassment with legal representation.

Donors include Witherspoon, Meryl Streep, Steven Spielberg and wife Kate Capshaw, and leading talent agencies that are under pressure to reform their practices in the aftermath of the Harvey Weinstein scandal.

Witherspoon told the New York Times the organization, which was founded in October, offered an opportunity for women to join forces in an industry that has typically been run by men.

“We’re finally hearing each other, and seeing each other, and now locking arms in solidarity with each other, and in solidarity for every woman who doesn’t feel seen, to be finally heard,” Witherspoon said.

While Time’s Up plans to structure itself as a decentralized organization with no formal hierarchy but established traditions, it plans to forge alliances with similarly focused organizations, among them 50/50 by 2020, a group that aims for equal representation in US boardrooms by 2020.

Producer and legal defense fund donor Katie McGrath said that many women had realized from early on that they needed to establish what they wanted to achieve, “and what was going to be required in order to shift and pivot from this horror to structural change”.