On the eve of President Donald Trump’s promised raids to round up undocumented immigrants, and as images of detainees in sometimes squalid cages continue emerging, Sen. Elizabeth Warren (D., Mass.) issued a stark warning Saturday — pledging in Philadelphia that if elected she would order her administration to investigate crimes committed in the name of immigration enforcement.

“To anyone out there who’s working in this system, understand: You abuse immigrants, you physically abuse immigrants, you sexually abuse immigrants, you fail to get the medical care that they need, you break a law of the United States of America,” Warren said at Netroots Nation, a national gathering of liberal activists at the Convention Center. “Donald Trump may be willing to look the other way, but President Elizabeth Warren will not.”

Warren, the most prominent of the four Democratic presidential candidates to visit the event, pledged that on her first day in office, she would “empower a commission in the Department of Justice to investigate crimes committed by the United States."

Her answer drew some of the most sustained cheers during a candidate forum in which questions about immigration received some of the most impassioned responses.

The focus at the liberal gathering showed how heavily the issue could weigh in the Democratic primaries and previewed what could be a striking contrast between Trump and Democrats in 2020.

“When I read stories about children who do not have clean clothes, who do not have access to water, who do not have access to healthy food while they are locked up by this president, it is something that is against humanity,” said Sen. Kirsten Gillibrand (D., N.Y.).

“This administration traffics in cruelty,” said Julian Castro, the former housing secretary in the Obama administration. He argued that Trump is betting “there are enough Americans out there whose fear and paranoia can be stoked in a way that they’re going to go out and vote for him.”

And Washington Gov. Jay Inslee called for changing the rules of the U.S. Senate so that Democrats could more easily pass legislation protecting “dreamers,” people brought to the country illegally as young children.

The charged debate loomed over the candidate forum, especially with the prospect of a sweeping immigration roundup planned to begin Sunday. Trump believes that the issue fires up his supporters and that Democrats, in response, have pushed so far left that they have hurt themselves with swing voters.

Protesters at Netroots interrupted Warren, holding up a sign reading “Legalize 11 million,” a reference to the estimated number of undocumented immigrants in the country, and “Reunify all families.”

Trump argues he is protecting the country from a wave of undocumented and potentially dangerous immigrants, and accuses Democrats of supporting “open borders,” pointing to some candidates’ calls for decriminalizing border crossings.

"Under President Trump, Pennsylvanians have seen major efforts to secure our southern border and put an end to the humanitarian crisis that has perpetuated the flow of crime and drugs into our country," said Christiana Purves, a spokeswoman for the Republican National Committee.

Democrats do not support open borders, though several of their presidential candidates have called for decriminalizing unauthorized border crossings and handling cases through civil courts. They also have decried what they see as inhumane conditions at the border, including the separation of children from their parents and people young and old held in filthy, crowded conditions, sometimes without soap or toothpaste.

The night before the candidates’ forum, Vice President Mike Pence visited one site where reporters saw some 400 men caged in a cell without enough space for some to lie down, no blankets to use on the concrete floor, and the foul smell of body odor in the air. Some said they had been held for more than 40 days and wanted to brush their teeth, though another site Pence visited was cleaner, according to pool reporters who accompanied him.

Pence said the conditions show that the crisis Trump has warned of is real.

Rep. Chrissy Houlahan visited the same crowded station in McAllen, Texas, on Saturday and, on Twitter, decried “deplorable” conditions for both migrants and the officers there.

“We in Congress must come together now and pass legislation that responsibly alleviates this crisis and prevents such inhumanity from happening on our soil in our name,” the Chester County Democrat tweeted.

Warren called for an overhaul that “starts with the premise that immigration is good for this country.”

She said she would expand legal immigration, create a path to citizenship for people in the country illegally but integrated into their communities, and reforming enforcement at the border.

“No great nation tears families apart. No great nation locks up children,” Warren said.

Gillibrand said she would end spending on for-profit prisons that detain children, allow asylum-seekers to live in communities while their cases are decided, and ensure that they have lawyers and “real judges” to hear their claims.

Castro called for breaking up the Immigration and Customs Enforcement (ICE) and has led the call to have unauthorized border crossing cases handled by civil courts. Inslee called “dreamers” some of “the most ambitious people in my state.”

Warren, who has surged in polls and in some surveys is now the second-place candidate, making her the leading alternative to former Vice President Joe Biden, received by far the most enthusiastic reception at the event, an annual gathering for grassroots liberals. This edition, the first in Philadelphia, drew some 3,700 people, according to organizers.

“Warren! Warren!” guests chanted as she came on stage. When she left the stage, a group of supporters chanted “Forty-six!” a reference to her potentially becoming the 46th president.

“Before she said a word … the whole feeling in the room changed and I thought, Oh, this is her room," said Leslie Sterling, 62, an Episcopal priest from Boston. "Maybe that’s why some of the other candidates, some of the other leading candidates didn’t want to be here, because they knew this was her room today.”

Most of the top Democratic candidates skipped the event.

Biden, a past attendee, campaigned in New Hampshire, as did New Jersey Sen. Cory Booker. California Sen. Kamala Harris was in Atlantic City, where she was scheduled to speak to a leadership conference of Omega Psi Phi, a black fraternity. South Bend, Ind., Mayor Pete Buttigieg planned events in Iowa. Sen. Bernie Sanders (I., Vt.) sent a surrogate who spoke earlier in the week.

Ronald Joseph, a 23-year-old activist from Philadelphia, said Sanders was his top pick for president, with Warren his second choice.

“I’m looking for candidates who have the best shot at beating Biden and then Trump," Joseph said. "A candidate that promises to maintain a status quo — it’s not enough for me because the status quo is killing people.”

Staff writer Lucia Geng contributed to this report.