President Bush on Thursday praised the coordinated government efforts to stop a terror plot to blow up airline flights from the United Kingdom to the United States, and said American flyers will be inconvenienced for a while in order for officials to keep them safe.

"The recent arrests that our fellow citizens are now learning about are a stark reminder that this nation is at war with Islamic fascists who will use any means to destroy those of us who love freedom, to hurt our nation," Bush said on a tarmac in Green Bay, Wis., before delivering a speech on the economy.

"Cooperation on this venture was excellent, the cooperation between U.K. and U.S. authorities and officials was solid, and the cooperation among agencies within our government was excellent," Bush said.

But even with the successful operation, the president said, "Obviously we still aren't completely safe, because there are people that still plot and people who want to harm us for what we believe in. It is a mistake to believe there is no threat to the United States of America."

The White House called the plot to blow up commercial flights in mid-air a "serious threat" to the United States and United Kingdom. Bush and British Prime Minister Tony Blair spoke overnight and last weekend as British intelligence discovered the scheme, which they'd been investigating for months, was imminent, Bush spokesman Tony Snow said. The two held phone calls while Blair is vacationing in the Caribbean and Bush is staying at his ranch in Crawford, Texas.

In an early morning press conference in Washington, Homeland Security Secretary Michael Chertoff also credited interagency coordination among the federal government and with British authorities for stopping the terror scheme that the secretary said is "suggestive of an Al Qaeda plot."

One senior U.S. counterintelligence official told FOX News that as many as 50 people were involved with this plot, which the official described as "the real deal." So far, 21 people have been arrested and British authorities say the "main players" are in custody.

Click here to read London Police Disrupt Terrorist Plot to Blow Up Aircraft in Mid-Flight.

"Today, air traffic is safe, and air traffic will remain safe precisely because of the measures we are adopting today," Chertoff said after announcing that the government has raised the threat level to red, the highest alert, for flights originating from Britain. This is the first time the level has gone to red since the color-coded system was put in place shortly after Sept. 11, 2001.

Other flights, including all domestic flights in the United States, are now on orange alert, the second highest security ranking, after British authorities accused the would-be suicide bombers with plotting to bring down flights run by United, Continental and American Airlines and British Airways. Passengers on American and United flights were casualties of the Sept. 11, 2001, terror attacks when four planes were hijacked and crashed.

The government last raised the terror risk to orange, or high, following the July 2005 subway bombings in London. It was lowered to yellow a month later, the elevated risk status that has been the norm since the system was created.

"The plot was to board international flights, potentially headed to the U.S., with bombs fashioned in a way that they would be in carry-ons, and blow them up in mid-air," one intelligence official said. The official said the terrorists had hoped to target flights to major airports in New York, Washington and California, all popular summer tourist destinations.

Chertoff said the plot involved the use of liquid explosives that could be carried on board disguised as beverages, electronic devices and other common objects.

In response, the U.S. government has banned all liquids and gels from flights, including toothpaste, makeup, suntan lotion. Baby formula and medicines were exempted, but will be tested before allowed on board. Airports have also added an additional layer of security at boarding gates.

Massachusetts Gov. Mitt Romney also announced he would deploy National Guard units to Logan Airport in Boston to help with security. Two of the flights on Sept. 11 originated from Logan.

Chertoff said even after the arrests, officials cannot conclude that the British-based plotters, whose plans carry the markings of Al Qaeda, have been completely thwarted.

"While this operation was centered in Great Britain, it was sophisticated, it had a lot of members and it was international in scope. This operation is in some respects suggestive of an Al Qaeda plot, but because the investigation is still under way, we cannot yet form a definitive conclusion," Chertoff said, adding, "We cannot assume that the threat has been completely thwarted or that we have fully identified and neutralized every member of this terrorist network."

Attorney General Alberto Gonzales said the operation could "potentially kill hundreds of innocent people."

FBI Director Robert Mueller also pointed at Al Qaeda. "This had the earmarks of an Al Qaeda plot," he said.

A senior U.S. counterterrorism official said authorities believe dozens of people were involved or connected to the overseas plot that was unraveled Wednesday evening. The plan "had a footprint to Al Qaeda back to it," said the official, speaking on condition of anonymity because of the sensitivity of the situation.

Chertoff said it was unclear whether the alleged plot was linked to the upcoming fifth anniversary of the Sept. 11 strikes. It also was not believed to be connected to the Egyptian students who disappeared in the United States more than a week ago before reaching a college they were supposed to attend in Montana. Three of the 11 have since been found and the FBI has said neither they nor the still-missing eight are believed to be a threat.

U.S. intelligence has been working closely with British officials on the investigation, which has been ongoing for months, a second official said.

The use of liquid explosives to blow up commercial flights is not a new idea. In the mid-1990s, terror leader Ramzi Youssef, who masterminded the first World Trade Center attacks in 1993, planned to put liquid bombs in contact lens solution containers to detonate on 12 Western airliners simultaneously. He was stopped before he could succeed with what is now called the Bojinga plan.

The metal detector and X-ray machines at airport security checkpoints cannot detect explosives. At many, but not all airport checkpoints, the TSA has deployed walkthrough "sniffer" or "puffer" machines that can detect explosives residue.

At U.S. Northern Command, the military headquarters established in response to the Sept. 11 attacks to improve coordination for the defense of U.S. territory, spokesman Sean Kelly said it would be inappropriate to discuss military operations.

After hearing of the plot, some lawmakers praised the government's actions while others criticized the Bush administration for not doing more.

"We must act decisively, aggressively, and innovatively, in a way that will protect Americans and our allies that have stood with us in this fight," said Senate Majority Leader Bill Frist, R-Tenn. "The increased security precautions being implemented around the country are appropriate and necessary. We must be on alert so that our nation does not suffer another attack like 9/11."

"This should serve as the latest, most serious evidence that we are in a war against a brutal enemy that intends to attack us over and over again in the most indiscriminate way," said Sen. Joe Lieberman, D-Conn., who lost a Democratic primary on Tuesday for what many say was his support for operations in Iraq. "The Department of Homeland Security is doing what it was created to do in the face of a terrorist threat: protecting the United States through the implementation of security measures, international cooperation, and coordination with the law enforcement and intelligence communities."

"While this terrorist plan may have been thwarted, I still am gravely concerned about gaping aviation security loopholes that continue to put passengers and crew members at risk," said Rep. Ed Markey, D-Mass., who has advocated 100 percent cargo screening on airline flights.

"While we are fortunate that today's attack appears to have been stopped by vigilant British authorities, today's announcement should be yet another wake-up call to the Bush administration to rapidly close the glaring loopholes that persist almost five years after 9/11," Markey said.'s Sharon Kehnemui Liss and The Associated Press contributed to this report.