WASHINGTON — From behind his desk in the Oval Office, US President Donald Trump signed a flurry of executive orders and decrees aimed at cracking down on immigration.

From an order tackling the definition of birthright citizenship, to an order declaring the illegal immigration at the border a national emergency, Trump swiftly made moves on his promises to tighten the US-Mexico border.

But some of those orders — particularly any order that aims to change the definition of birthright citizenship — are likely to face steep legal opposition.

Trump had previously vowed that the "destructive" policies of the Biden White House would be gone "within five minutes".

Hours earlier, thousands of would-be migrants had their immigration appointments canceled after the new administration scrapped CBP One, an app used to book appearances at ports of entry at the border.

In his inaugural address, Trump vowed that "all illegal entry will be halted" and that millions of "criminal aliens" will be deported.

He also signed an order declaring Mexican drug cartels terrorist organizations.

At a previous event at Washington's Capital One Arena, Trump formally revoked nearly 80 executive actions — which he described as "radical" — from the Biden administration.

"I have no higher responsibility than to defend our country from threats and invasions," Trump said in his inaugural address at the US Capitol earlier in the day.

As part of the broader plan, Trump administration officials said the president would direct the Department of Defense to "seal the border" and devote additional resources and personnel, including counter-drone capabilities.

Banner titled 'Trump's Inauguration' with red and purple stripes and white stars

Earlier, incoming Trump administration officials said that the moves amounted to the creation of "common sense immigration policy".

Although the details of the order are not yet known, officials have said that Trump plans to end birthright citizenship, meaning that the children of undocumented migrants living in the US will no longer automatically be considered US citizens.

Birthright citizenship, however, is enshrined in the US constitution and would require a two-thirds vote in both chambers of Congress to change. The official provided no further detail on how Trump plans to accomplish this.

The new administration also moved to swiftly scrap CBP One, a mobile application used by migrants to book appointments to appear at a port of entry.

Biden administration officials had credited the app with helping reduce the number of detentions at the border since it was first introduced in January 2023. It was the only legal pathway to request asylum at the US-Mexico border.

Now, the Customs and Border Protection website notes that the app is "no longer available".

App users also now are shown a message noting that "existing appointments scheduled through CBP One are no longer valid".

According to CBS, the BBC's US partner, the Biden administration had scheduled roughly 30,000 appointments via CBP One for migrants to enter the US in the next three weeks.

Other estimates had suggested that as many as 270,000 migrants were in Mexico waiting for an opportunity to enter the US using CBP One.

In the Mexican border city of Tijuana, some migrants reported feeling defeated and deflated after learning of CBP One's demise.

"I hope God touches his [Trump's] heart," said Oralia, a Mexican woman who fled cartel violence in her home state along with her epileptic son. "We really do need the help."

She had been waiting for an appointment through CBP One for seven months. — BBC