President Donald Trump’s order banning citizens from 12 countries from entering the United States has come into effect amid rising political tensions over his administration’s harsh anti-immigration policies.

The measure, announced by Trump last week as necessary to prevent the importation of “terrorists”, took effect on Monday. The clampdown comes amid chaotic scenes on the streets of Los Angeles as crowds of protesters battled with police and National Guard troops following a wave of arrests by immigration authorities.

The divisive order revives similar measures rolled out during Trump’s first term, as travellers from several, mostly Muslim, countries were blocked from entering the US. Many of the countries affected by the new order are afflicted by war and large-scale displacement.

No visible disruption was immediately discernible at Los Angeles International Airport in the hours after the new ban took effect, according to the Associated Press news agency.

The order applies to citizens of Afghanistan, Chad, Eritrea, Equatorial Guinea, Haiti, Iran, Libya, Myanmar, the Republic of the Congo, Somalia, Sudan and Yemen.

Heightened restrictions were also placed on people from Burundi, Cuba, Laos, Sierra Leone, Togo, Turkmenistan and Venezuela.

Trump said the countries subjected to the most severe restrictions were determined to harbour a “large-scale presence of terrorists”, fail to cooperate on visa security and have an inability to verify travellers’ identities. These countries were also inadequate in keeping records of criminal histories and had high rates of visa overstays in the US, according to Trump.

The new ban does not revoke visas previously issued to people from countries on the list, according to guidance issued on Friday to all US diplomatic missions.

But it remains unclear how those rules will be implemented at the ports of entry. During Trump’s first term, a similar travel ban resulted in confusion and disrupted travel.

In announcing the new restriction last week, Trump said the measure was spurred by a recent “terrorist attack” on Jewish people in the US state of Colorado.

The group had been protesting in solidarity with captives held in Gaza when they were assaulted by an Egyptian man that the White House said had overstayed his visa.

That attack, Trump said, “underscored the extreme dangers posed to our country by the entry of foreign nationals who are not properly vetted”.

He warned that new countries could be added “as threats emerge around the world”.

Egypt is not among the states affected by the ban or increased scrutiny.

Volker Turk, United Nations high commissioner for human rights, warned that “the broad and sweeping nature of the new travel ban raises concerns from the perspective of international law”.

‘Unlawful assembly’ zone

The ban comes amid protests in the city of Los Angeles against immigration raids, carried out as part of Trump’s hardline policy.

Over the weekend, Trump ordered the deployment of 2,000 members of the National Guard to Los Angeles County to quell the protests, bypassing the authority of the governor of California and sending tensions spiking.

Thousands of protesters flooded the streets in response on Sunday, blocking a major freeway and setting fire to cars.

Law enforcement responded with tear gas, rubber bullets and flashbangs in an effort to disperse the crowds.

Early on Monday, authorities declared downtown Los Angeles an “unlawful assembly” zone and ordered the area cleared.

California Governor Gavin Newsom has formally requested that the Trump administration rescind the order deploying National Guard troops to the city.

The order is believed to be the first time in 60 years that a president has deployed a state’s National Guard without the governor’s consent.

The last instance was in 1965, when President Lyndon B Johnson used troops to protect predominantly Black demonstrators during the civil rights movement in Alabama.

However, officials from the Trump administration, aware that aggressive immigration enforcement is popular among his base, have been eager to declare that they will clamp down on what they have labelled an “insurrection” and “migrant invasion”.