The twin strikes, the first such joint effort in more than 60 years, will add to the economic damage from the writers' walkout that started on May 2, increasing the pressures facing the multibillion-dollar industry as it struggles with changes to its business.
In New York City and Los Angeles, actors marched outside the offices of Warner Bros, Paramount and other major studios, chanting and waving signs.
"I actually make less money working in film than I did in the year 1990," said Andrea Salloum, an actor who joined scores of people picketing the Netflix offices in Los Angeles.
"It's really scary with the artificial intelligence."
Unions across the United States have been taking harder lines in negotiations over better wages and benefits, from railroad employees and airline pilots to Amazon.com and Starbucks workers.
The largest US car workers' union and UPS employees are also in the midst of contract talks.
Both SAG-AFTRA, Hollywood's largest union, and the Writers Guild of America (WGA) are demanding increases in base pay and residuals, or fees paid from streaming television, plus assurances that their work will not be replaced by artificial intelligence (AI).
Although SAG-AFTRA's ranks include the most famous, and wealthiest, Hollywood movies stars, the picket lines on Friday were filled with less familiar faces that make up the majority of the union's 160,000 members.
Many said it was harder to make a living than even a few years ago.
Tatum Price said it felt like "something purposeful" to be among the hundreds of film and television actors picketing the neighbouring Los Angeles offices of Sony and Amazon.
Her biggest concerns are the decline of residuals as more productions have ended up on streaming services such as Hulu and Netflix.
"A lot of people will watch their favourite episodes over and over again but we're not getting the same residuals that you'd get from network television shows and commercials," Price said.
The actors' union announced the strike after failing to reach a deal with studios, including Walt Disney Co and Netflix Inc.
Fran Drescher, former star of The Nanny TV show and the president of SAG-AFTRA, said the contract offer from the studios was very far apart from the actors' demands, adding that the studios wasted a 12-day extension in talks.
She linked the actors' fight to a broader surge in labour union activity in the US.
"If we don't take control of this situation from these greedy megalomaniacs, we are all going to be in threat of losing our livelihoods," Drescher said from the picket line outside the Netflix offices.
The Alliance of Motion Picture and Television Producers (AMPTP), the group that negotiates on behalf of studios, said it had offered significant gains to union members.
They included the highest percentage increase in minimum pay levels in 35 years and "groundbreaking" protections around the use of actors' images by generative AI, the organisation said.
"Rather than continuing to negotiate, SAG-AFTRA has put us on a course that will deepen the financial hardship for thousands who depend on the industry for their livelihoods," the AMPTP said.
The strike by roughly 11,500 writers has sent late-night television talk shows into endless reruns, disrupted most production for the northern hemisphere autumn TV season and halted work on big-budget movies.
The actors' walkout will shut down the studios' remaining US-based productions of film and scripted television and hamper many overseas shoots.
Many streaming services have yet to turn a profit after companies spent billions of dollars on programming to try and attract customers.
The United Kingdom's main entertainment industry union, Equity, said it backed its US counterpart and would be bringing up many of the same issues in its own contract negotiations over the next 12 months.
