From the White House, the Pentagon, world leaders and Muslim advocacy groups there has come a barrage of criticism for billionaire Donald Trump after the leading U.S. Republican presidential candidate called for a "total and complete shutdown" of Muslims entering the United States.

White House spokesman Josh Earnest on Tuesday called Trump's rhetoric "offensive and toxic," saying his plan "disqualifies him from serving as president"  because he would be violating the U.S. Constitution if he implemented it, should he win the country's 2016 presidential contest.

At the Pentagon, spokesman Peter Cook highlighted the military's partnerships with Muslim nations as well as its own Muslim troops, and noted the security implications of anything that boosts Islamic State claims that the U.S. is at war with Islam.

"Anything that bolsters ISIL's narrative and pits the United States against the Muslim faith is certainly not only contrary to our values, but contrary to our national security," he said, using an acronym for the militant group.

House Speaker Paul Ryan said Trump's proposal to bar Muslims until U.S. leaders can "figure out what is going on" about possible new terror attacks is not what the Republican party or the country stand for.

"Freedom of religion is a fundamental constitutional principle.  It's a founding principle of this country," Ryan said Tuesday.

Across the Atlantic, British Prime Minister David Cameron called Trump's plan "divisive, unhelpful and quite simply wrong," while U.S. Secretary of State John Kerry told reporters in Paris that Trump's comments are "not constructive."

'Islamophobia, xenophobia'

Farhan Haq, deputy spokesman for U.N. General-Secretary Ban Ki-moon, in responding to Trump's latest comments, said, "We do not believe that any kind of rhetoric that relies on Islamophobia, xenophobia, any other appeal to hate any groups, really should be followed by anyone."

FILE - White House press secretary Josh Earnest at
FILE - White House press secretary Josh Earnest at a White House press briefing in Washington, Sept. 18, 2015. Earnest called Trump's calls for a ban on Muslims entering the country "offensive and toxic."

Keysar Trad, chairman of the Sydney-based Islamic Friendship Association of Australia, called Trump's comment "a desperate statement by a desperate man who knows that he's clutching at straws and has no chance of winning the election. So he's trying to win it off the back of the Islamophobia industry."

In the U.S., Trump's political opponents widely condemned his proposal and legal experts told news outlets that a ban on Muslims entering the country would be unconstitutional.

US Constitution's 1st Amendment

"Donald Trump is unhinged," Republican presidential contender Jeb Bush, a former Florida governor and son and brother of two former U.S. presidents, said on Twitter, "His 'policy' proposals are not serious."

Another of Trump's Republican opponents, South Carolina Senator Lindsey Graham, described Trump's proposal to ban Muslims as "un-American," and said, "He is helping the enemy of this nation, by assisting Islamic State to recruit more fighters."

The Republican leader of the House of Representatives, Speaker Paul Ryan, denounced the Trump idea, saying it "is not what this party stands for and more importantly not what this country stands for."

WATCH: House Speaker Paul Ryan reacts to Trump comment on Muslim ban

Ibraham Hooper, director of the Council on American-Islamic Relations, had an especially blistering response to Trump's demand.

"We're entering into the realm of the fascist now," Hooper said. "It should be disturbing not only to American-Muslims, but it should be disturbing to all Americans that the leading Republican presidential candidate would issue essentially a fascist statement like this."

US Constitution

Noted U.S. constitutional expert Laurence Tribe, a Harvard Law professor, told MSNBC that Trump's proposal would violate the U.S. Constitution's ban on religious tests and granting all citizens equal rights.

Moreover, Tribe said, the plan would be "impossible to administer" and "stupidly play into the hands of extreme Islamic terrorists."

Another law professor, Richard Friedman of the University of Michigan, told The Washington Post that Trump's idea is "blatantly unconstitutional if it excludes U.S. citizens (from re-entering the U.S. after trips to other countries) because they are Muslims. It's ridiculous."

Trump supporters yell at passing protesters (unsee
Trump supporters yell at passing protesters (unseen), who are against Trump's policies, during a rally by U.S. Republican presidential candidate Donald Trump aboard the USS Yorktown Memorial in Mount Pleasant, S.C., Dec. 7, 2015.

Trump, a billionaire real estate mogul who leads presidential preference surveys of Republicans, announced the plan Monday in a statement, then expanded on it at a political rally in South Carolina.

He called it "common sense" and said that "we have no choice" in the wake of last week's attack at a California government center that killed 14 people and was carried out by two Muslims, a husband-and-wife team investigators say had been "radicalized."

On Tuesday morning news shows in the U.S., the flamboyant Trump continued to defend the proposal and condemned President Barack Obama's efforts in leading the fight against Islamic State militants in the Middle East.

"We are now at war," Trump told ABC's Good Morning America. "We have a president who doesn't want to say that."

In a contentious interview on CNN, Trump referenced the 2001 terrorist attacks on the U.S., saying, "You're going to have many more World Trade Centers if you don't solve it. They want our buildings to come down. They want our cities to be crushed. They are living in our country and many are outside of our country."

FILE - American Shi'ite Muslims march to the White
American Shi'ite Muslims march to the White House to protest against the Islamic State group, in Washington, D.C., Dec. 6, 2015.

'Hatred toward Americans'

In announcing the plan, Trump alleged that polling data shows "hatred toward Americans by large segments of the Muslim population."

"Until we are able to determine and understand this problem and the dangerous threat it poses, our country cannot be the victims of horrendous attacks by people that believe only in jihad, and have no sense of reason or respect for human life," Trump said.

Virulent statements against Muslims are nothing new for Trump, who has called on the government to monitor mosques, and has refused to rule out his earlier proposal to enter the names of Muslims in America into a database.

Trump's proposal came a day after Obama urged Americans during a nationally televised address not to turn against Muslims in the wake of the terrorist attacks.

Mary Alice Salinas at the White House and Margaret Besheer at the United Nations contributed to this report.

WATCH: International reaction to Trump's Muslim ban comment