Among the hotly debated issues are the president's use of executive power and whether there really is an emergency.
"Clearly, this is one more effort at a power grab by the president who has gone outside the bounds of the law to try to get what he failed to achieve in the constitutional manner," he said.
The event, sponsored by the activist group MoveOn, attracted people with signs marching the square. They chanted slogans like, "Say it loud. Say it clear. Immigrants are welcome here."
Hundreds of protesters chanted outside the White House on Monday, the annual U.S. President Day holiday, as part of the nationwide demonstrations.
Cassandra Boulanger told Xinhua that "I think the so called emergency is misused and exaggerated by president of United States. It is not existing emergency."
Customs and Border Protection officers apprehended close to 80,000 people on the U.S.-Mexico border during the 2018 fiscal year, according to federal data.
"There's nothing wrong with it. Other presidents have done it. Why weren't they protesting when Obama put the country on national emergency?" she said.
"President Trump has a lot of policies; I'm here today to stand up for the national emergency," said Karen, a local retiree.
There are also a few Trump supporters in the rally.
U.S. politicians should "work together for decency and positivity," instead of creating more divisions, Newman told Xinhua.
Garcia vowed to explore both legislative and legal actions in the days and months to come.
In New York City, hundreds of people rallied in Union Square to protest against Trump's declaration and decry Trump's proclamation as "undemocratic and anti-immigrant."
Jesus "Chuy" Garcia, a U.S. congressman from Illinois, told the crowd that at least 12 states have already joined legal efforts led by the Attorney General of California, "suing (Donald) Trump to undo the phony emergency that he has declared at the border."
There has been an "increasing number of people crossing" and "a huge increase in drug deaths" since 800, he claimed.
The back-and-forth between the White House and Congressional Democrats is distracting them from doing what's really important for the country, Dick Newman, a retiree from Annapolis, Maryland, lamented.
Hal Ponder, a Washington, D.C. resident and former Congress employee, told Xinhua that he believes "there is no emergency" at the southern border and that the president is "making this up" to get around Congress to push for his signature campaign promise.
That's an increase from the prior year, but less than in 2016. And recent annual figures are the lowest they've been in nearly four decades, according to local media reports.
"End the fake emergency now," a banner read. "We stand with immigrants and asylum seekers," another claimed.
Trump signed a national emergency on Friday to build the U.S.-Mexico border wall and push for his signature campaign promise. The move gave the president power to bypass Congress to get access to money, but it sparked a new round of legal and partisan battles almost immediately.
The two-month fight over the funding of the border wall between the White House and congressional Democrats has led to the record-breaking 35-day federal government shutdown which ended late January.
White House senior policy adviser Stephen Miller defended Trump in an interview on "Fox News Sunday," insisting the emergency is real.
The emergency declaration gives the president power to bypass Congress and redirect several billions of dollars for his border wall pledge, triggering a new round of legal and partisan battle.
"It's all political. It's not real," said Ponder at Lafayette Square in front of the White House.
Liliana, a student from New Jersey, was among the protesters. "It's heartbreaking because it (Trump's policy) affects my family a lot," said Liliana, whose parents are first-generation immigrants. Enditem
Local media said at least two lawsuits by watchdog groups are challenging the legality of the declaration, and California's Attorney General Xavier Becerra said Sunday he would "definitely and imminently" file a suit.
"The current emergency is our health care system, naturalization of other immigrants, offering help to all minority groups by the administration. Especially gun violence as well. So no, this is not realistic emergency," said Boulanger, a staff member of a local museum.
More than 80 people took part in the rally at Chicago's downtown Federal Plaza, protesting against what they call "a phony national emergency as an end run around congress."
WASHINGTON, Feb. 18 (Xinhua) -- Demonstrations were held Monday in Washington, New York, Chicago and dozens of other U.S. cities against President Donald Trump's emergency declaration aimed to secure funding for the highly controversial U.S.-Mexico border wall.