Several Democratic lawmakers on Sunday paid a "surprise Father's Day visit" to an Immigration and Customs Enforcement (ICE) detention center in New Jersey, capping off a week of heightened tensions surrounding the issue of migrant family separations.

Reps. Hakeem Jeffries Hakeem Sekou JeffriesDemocratic leaders: Supreme Court fight is about ObamaCare Pelosi: House will stay in session until agreement is reached on coronavirus relief Races heat up for House leadership posts MORE (N.Y.), Jerrold Nadler (N.Y.), Adriano Espaillat Adriano de Jesus Espaillat CabralLawmakers call for small business aid at all levels of government The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy On the Money: Administration to ban TikTok, WeChat | House moves toward bill to avoid government shutdown | Coronavirus relief bills boosted GDP, CBO says MORE (N.Y.), Carolyn Maloney Carolyn Bosher MaloneyPelosi, Democrats unveil bills to rein in alleged White House abuses of power Government watchdog recommends creation of White House cyber director position Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence MORE (N.Y.), Frank Pallone Jr. Frank Joseph PallonePharma execs say FDA will not lower standards for coronavirus vaccine Dem chairmen urge CMS to prevent nursing homes from seizing stimulus payments Federal watchdog finds cybersecurity vulnerabilities in FCC systems MORE (N.J.), Albio Sires Albio B. SiresWe can't lose sight of Ortega's abuses in Nicaragua Hispanic Caucus requests meeting with private detention center CEOs New Jersey Rep. Bill Pascrell wins Democratic primary MORE (N.J.) and Bill Pascrell William (Bill) James PascrellRep. Bill Pascrell named chair of House oversight panel Trump says people 'in the dark shadows' are controlling Biden Democrats tear into Trump's 'deep state' tweet: His 'lies and recklessness' have 'killed people' MORE (N.J.) arrived around 9 a.m. at the Elizabeth, N.J., center, which houses migrants who were separated from their families after illegally crossing the border.

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According to Jeffries, the lawmakers were blocked from entering the facility after meeting initial resistance from ICE staff.

Here w/ six other members of Congress including ⁦@RepJerryNadler⁩ ⁦⁦@RepEspaillat⁩ at ICE detention facility in Elizabeth, NJ making surprise Father’s Day visit. They are refusing to let us in. We will #Resist #KeepingFamiliesTogether pic.twitter.com/gZ3zzKCDos — Hakeem Jeffries (@RepJeffries) June 17, 2018

Maloney tweeted that the group was allowed in more than an hour later after "banging on the door" and telling ICE staff and local police that they would not move until they were let in.

"After being made to wait over an hour. After having to deal with local PD. After banging on the door and making it clear that we WILL NOT MOVE until we're allowed in, ICE is finally granting us access to see individuals who have been separated from their families," Maloney tweeted.

After being made to wait over an hour. After having to deal with local PD. After banging on the door and making it clear that we WILL NOT MOVE until we're allowed in, ICE is finally granting us access to see individuals who have been separated from their families. pic.twitter.com/coXJNdxsLP — Carolyn B. Maloney (@RepMaloney) June 17, 2018

“We are now inside speaking with detainees who were separated from their children when they sought asylum in the US from terrible violence and danger at home,” Nadler tweeted.

We are now inside speaking with detainees who were separated from their children when they sought asylum in the US from terrible violence and danger at home. We will be addressing the media outside shortly. #FamilesBelongTogether #EndFamilySeparation https://t.co/CfvQJVxDXG — (((Rep. Nadler))) (@RepJerryNadler) June 17, 2018

"Protests outside the ICE detention center in Elizabeth continue to swell as I and my House colleagues tour the facility. Americans are fed up with the Trump administration's cruelty," Pascrell tweeted.

Protests outside the ICE detention center in Elizabeth continue to swell as I and my House colleagues tour the facility. Americans are fed up with the Trump administration's cruelty. #FathersDay #FamiliesBelongTogther pic.twitter.com/ZeKPm6h44D — Bill Pascrell, Jr. (@BillPascrell) June 17, 2018

ICE said in a statement that media tours of the facilities require advance notice.

“The Congressmen were allowed to visit several detainees. Media tours need an advanced written request to ICE," the agency said.

Sunday's visit in New Jersey comes amid a growing campaign from Democrats and some Republicans to tour detention facilities housing migrant children who have been separated from their family members.

Sen. Jeff Merkley Jeffrey (Jeff) Alan MerkleyThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Sunday shows - Trump team defends coronavirus response Oregon senator says Trump's blame on 'forest management' for wildfires is 'just a big and devastating lie' MORE (D-Ore.) is scheduled to lead a group of congressional lawmakers on a tour of a Brownsville, Texas, facility later Sunday.

Rep. Will Hurd William Ballard HurdHillicon Valley: Oracle confirms deal with TikTok to be 'trusted technology provider' | QAnon spreads across globe, shadowing COVID-19 | VA hit by data breach impacting 46,000 veterans House approves bill to secure internet-connected federal devices against cyber threats House Democrats' campaign arm reserves .6M in ads in competitive districts MORE (R-Texas) visited a similar facility in Tornillo, Texas, on Friday night.

The tours come amid congressional pushback against the Trump administration's "zero tolerance" policy of separating migrant families who cross the border illegally.

Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE announced last month that the Justice Department would enact the policy, which aggressively prosecutes adults attempting to cross the U.S.–Mexico border illegally. At the time, Sessions acknowledged the process could lead to children being separated from their parents upon being apprehended.

Trump administration officials have repeatedly defended the policy, arguing that it acts as a deterrent against illegal immigration.

Democrats and Republicans, however, have spoken out against the practice, calling it cruel, immoral and un-American.

The Trump administration has indicated it will not unilaterally end the practice. Instead, the president and other White House officials have blamed Democrats for refusing to negotiate new immigration laws.

Updated at 12:56 p.m.