Thousands take to streets in Women's March on anniversary of Trump's 1st year.

People take part in the Women's March in New York City, Jan. 20, 2018.

From New York City to Los Angeles and many cities in between, thousands of women and their allies took to the streets Saturday, vowing to show up at the polls this year for midterm elections amid outrage over President Donald Trump's agenda.

The main event for the 2018 Women's March, entitled "Power to the Polls," will take place Sunday in Las Vegas, where organizers will launch a national voter registration and mobilization initiative. Hundreds of other anniversary marches and events will be held elsewhere in the nation -- and around the world -- on both Saturday and Sunday.

Joyce Pleva and her two adult daughters, Elizabeth and Jamie, were among those marching in New York City. The family participated in the Women's March last year in Washington, D.C., where a sea of women wearing pink "pussy hats" gathered to protest a day after Trump was inaugurated as the 45th president of the United States.

"Last year, there was a feeling of fear and objection to the election," Elizabeth Pleva told ABC News, while waiting for the march to commence on Manhattan's Upper West Side. "This year, there are so many more reasons we are here. DACA, the ACA, the environment and [President Trump's] inability to govern."

Throughout his first year in office, Trump has tried to unravel the ACA, or the Affordable Care Act, which was his predecessor's signature health care reform law.

Obamacare, as it's also known, was critical to the Pleva family while Elizabeth and Jamie's other sister battled breast cancer. She recently died from the illness.

"We are especially concerned about the ACA ... I have the gene that leads to breast cancer," Jamie Pleva said. "What would I do without the preexisting condition [coverage]?"

People take part in the Women's March in Manhattan in New York, Jan. 20, 2018.

Jamie Williams, originally from Alabama, brought her 7-year-old son, James, to march with her in Manhattan.

"I want him to be exposed to this," Williams told ABC News. "I also bring him with me every time I vote, so the vote counts for both of us."

Williams said she participated in the Women's March in Birmingham, Alabama, last year.

"I think it's really important to march last year and this year," she said. "We're not backing down after Trump was elected."

People participate in the second annual Women's March in Washington, D.C., Jan. 20, 2018.

The anniversary marches in New York City and Washington, D.C., are expected to be among the largest Saturday. The march in New York City starts at 12:30 p.m. Eastern Time from Central Park West and 72nd Street, following a pre-march rally at Central Park West and 61st/62nd Streets at 11:30 a.m. The march in D.C. starts at the Lincoln Memorial Reflecting Pool at 11 a.m. Eastern Time.

Organizers chose Nevada to host the main event this year because it "was rocked by the deadliest mass shooting in modern U.S. history, recent sexual assault allegations against elected officials and has become a battleground state that will shape the Senate in 2018," according to the Women's March website.

"Women's March has created a powerful movement that has ignited thousands of activists and new leaders," Tamika Mallory, co-president of Women’s March, said in a statement ahead of Sunday. "In 2018, we must turn our work into action ahead of the midterms. This new initiative will address voter registration and voter suppression head on. We marched for justice in D.C., we created our plan in Detroit and now we’re bringing the power of the polls to Nevada."

A protester in Morristown, N.J., holds a sign focusing on the upcoming midterm elections, Jan. 20, 2018.

There are also marches and events planned in dozens of cities abroad, including Buenos Aires, Kampala, London, Bangkok, Beijing and Sydney.