Donald Trump has defied fierce criticism to announce that he is using emergency powers to bypass Congress and pursue the building of a wall on the US-Mexico border.

At a combative, rambling and at times incoherent press conference in the White House, the US president insisted he had no choice but to declare a national emergency to stop illegal immigrants spreading crime and drugs.

Yet Trump admitted that he did not “need” to take the step now and was only doing so for speed. Opponents seized on the remark to accuse him of falsehoods and fearmongering for political ends, describing the move as “unlawful” and a violation of the US constitution.

Speaking in the White House rose garden, Trump noted that the National Emergencies Act of 1976 gave presidents leeway to declare an emergency and several have done so. “There’s rarely been a problem,” he said. “Nobody cares. I guess they weren’t very exciting. But nobody cares. They sign it for far less important things in some cases, in many cases.”

A list compiled by the Brennan Center says Bill Clinton declared 17 national emergencies, George W Bush 13 and Barack Obama 12, but nearly all were for crises that emerged overseas. Trump added: “We’re talking about an invasion of our country, with drugs, with human traffickers, with all types of criminals and gangs.”

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Trump then asked two women whose family members had been killed, allegedly by undocumented immigrants, to stand and hold up photos of their late loved ones to assembled staff and media. He went on to recycle gory descriptions of violent criminal gangs and warned of a “virtual invasion” that requires urgent remedy.

But later, taking questions from reporters, he muddied the waters about his motives. “I went through Congress. I made a deal. I got almost $1.4bn when I wasn’t supposed to get $1. Not $1! He’s not going to get $1! Well, I got $1.4bn, but I’m not happy with it … I want to do it faster. I could do the wall over a longer period of time. I didn’t need to do this but I’d rather do it much faster.”

Trump went on: “I don’t have to do it for the election. I’ve already done a lot of wall for the election: 2020. And the only reason we’re up here talking about this is because of the election, because they want to try and win an election which it looks like they’re not going to be able to do.”

The Democratic House speaker, Nancy Pelosi, tweeted: “He admits it’s a # FakeTrumpEmergency. Hear him say it: “I *didn’t need* to do this, but I’d rather do it much faster.”

The admission could complicate forthcoming legal battles if, as is widely expected, the declaration is challenged in court by states and others who stand to lose federal money or claim that Trump is abusing his authority.

The White House said the national emergency would take about $3.6bn from the Pentagon’s military construction fund. Executive action will divert around $2.5bn from the Pentagon’s drug interdiction programme and $600m from the treasury’s drug forfeiture fund.

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Trump acknowledged the high likelihood of a legal fight, and one that he would almost certainly lose in the lower courts. “I expect to be sued,” he said. “Sadly, we’ll be sued and sadly it will go through a process and happily we’ll win.”

He went on in a mocking, sing-song voice: “I’ll sign the final papers ... and we will have a national emergency, and we will then be sued, and they will sue us in the ninth circuit ... and then we’ll end up in the supreme court and hopefully we’ll get a fair shake and we’ll win in the supreme court just like the [travel] ban.”

Trump’s decision came after weeks of wrangling over his signature campaign promise that led to a record 35-day partial government shutdown, which proved politically damaging and hurt his approval rating.

On Thursday, in a rare display of bipartisanship, Congress approved a border security compromise deal to avert another shutdown. The Senate passed the legislation 83-16, while the House of Representatives followed with a 300-128 tally.

But the bill devotes about $1.4bn for border barriers, well below the $5.7bn that Trump demanded to fund just a quarter of the 200-plus miles he is seeking.

Many observers suspect that, after weeks of debate, Trump was influenced by conservative commentators on Fox News and elsewhere, who warned that signing the bill alone would be tantamount to surrender.

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In a joint statement, Pelosi and the Senate minority leader, Chuck Schumer, both Democrats, said: “The president’s unlawful declaration over a crisis that does not exist does great violence to our constitution and makes America less safe, stealing from urgently needed defense funds for the security of our military and our nation.

“This is plainly a power grab by a disappointed president, who has gone outside the bounds of the law to try to get what he failed to achieve in the constitutional legislative process.”

Pelosi and Schumer said they would use “every remedy available” to oppose Trump’s declaration. New York state’s attorney general, Letitia James, said her office would challenge Trump in court. “We won’t stand for this abuse of power & will fight back with every legal tool at our disposal,” James wrote on Twitter.

The House judiciary committee announced it would investigate the declaration and call a hearing, and by late Friday, the not-for-profit group Public Citizen had already filed a complaint against Trump. The American Civil Liberties Union and officials in California also said they planned to file suits.

Some Republicans have also criticised the move as setting a dangerous precedent, which could embolden a future Democratic president to declare emergency for a pet cause such as climate change or gun control.

Additional reporting by Sam Levin