What is the 1798 law that Trump used to deport migrants?

More than 200 Venezuelans alleged by the White House to be gang members have been deported from the US to a notorious mega-jail in El Salvador.

Out of the 261 people deported, 137 were removed under the Alien Enemies Act, a senior administration official told CBS News, the BBCs US partner.

This broad, centuries-old law was invoked by President Donald Trump. He accused Venezuelan gang Tren de Aragua (TdA) of "perpetrating, attempting, and threatening an invasion or predatory incursion" on US territory.

The move has been criticised by rights groups, and came despite a temporary block issued by a judge. The White House said the judges order itself was not lawful and was issued after the group was deported.

The Alien Enemies Act grants the president of the United States sweeping powers to order the detention and deportation of natives or citizens of an "enemy" nation without following the usual processes.

It was passed as part of a series of laws in 1798 when the US believed it would enter a war with France.

The act states that "whenever there shall be a declared war [...] or any invasion or predatory incursion shall be perpetrated, attempted, or threatened" against the US, all "subjects of the hostile nation or government" could be "apprehended, restrained, secured and removed, as alien enemies".

The act has only been previously used three times - all during times of conflict involving the US.

It was last invoked in World War Two, when people of Japanese descent - reportedly numbering about 120,000 - were imprisoned without trial. Thousands were sent to internment camps.

People of German and Italian ancestry were also interned during that time.

Before that, the act was used during the War of 1812 and World War One.

Though this is the first time the act has been used by Trump, it is not the first time he has mentioned it.

At his inaugural address in January, he said he would invoke the act to "eliminate the presence of all foreign gangs and criminal networks bringing devastating crime to US soil".

In his proclamation on Saturday, Trump invoked the wording of the act by accusing TdA of threatening an "invasion" against the US. He declared its members "liable to be apprehended, restrained, secured, and removed as alien enemies".

Trumps decision has been criticised by rights groups. The American Civil Liberties Union (ACLU) sued to stop the removals on the grounds that the US was not at war.

Speaking to BBC News on Sunday, Lee Gelernt, a lawyer with the ACLU, said: "Theres no question in our mind that the law is being violated."

A federal judge attempted to stop the use of the law to carry out the deportations, but the White House said this had had "no lawful basis", and that the removals had already taken place.

Reacting to a news article covering the judges order, El Salvadors President Nayib Bukele wrote on social media: "Oopsie... Too late."

Venezuela criticised Trumps use of the act, saying it "unjustly criminalises Venezuelan migration" and "evokes the darkest episodes in the history of humanity, from slavery to the horror of the Nazi concentration camps".

Katherine Yon Ebright, counsel at the Brennan Center for Justice, said in a statement that Trumps use of the Alien Enemies Act was illegal.

"The only reason to invoke such a power is to try to enable sweeping detentions and deportations of Venezuelans based on their ancestry, not on any gang activity that could be proved in immigration proceedings", she added.