(CBS/AP) British authorities said Thursday they had thwarted a major terrorist plot to simultaneously blow up several aircraft heading to the U.S. using explosives smuggled in hand luggage, averting what police described as "mass murder on an unimaginable scale."

Twenty-one suspects are in custody, after a series of arrests overnight.

The U.S. issued its highest terrorism alert ever, red, for commercial flights from Britain to the United States and raised the threat level for all domestic and international flights. All other flights, including all domestic flights in the United States, were put under an orange alert, one step below the highest level.

"No liquids or gels of any kind will be permitted in carry-on baggage. Such items must be in checked baggage. This includes all beverages, shampoo, sun tan lotion, creams, toothpaste, hair gel and other items of similar consistency," a Homeland Security statement cautioned passengers.

Homeland Security Secretary Michael Chertoff said the terrorists planned to bring various bomb components in a benign state aboard the planes and combine them once the planes were aloft to create and detonate explosive devices. Sources tell CBS News correspondent Jim Stewart that these chemical bombs would have been set with timers to go off simultaneously.

Stewart reports that the flights from London to New York were targeted, and involved flights on U.S. carriers heading to John F. Kennedy, Washington Dulles, Miami International and Los Angeles International airports aboard U.S. carriers.

U.S. counterterrorism officials tell CBS News correspondent Sheila MacVicar that United, American and Continental Airlines — all U.S. carriers — were targets of the plot.

"This is not a case of wanna-bes fantasizing about an attack. We believe it was the real deal. To target multiple airlines with such a plan requires a sophistication that strongly points to al Qaeda," a senior U.S. official told Stewart.

White House spokesman Tony Snow said President Bush has been "fully informed of the counter-terror and intelligence operation for several days."

"The president, prime minister and their key intelligence, law enforcement and security officials have been in close and regular contact. And as you would expect, he has spoken to Prime Minister Blair about these developments in recent days," Snow said Thursday.

Chertoff said the plot is "suggestive of an al Qaeda plot," and that the plotters were in the final stages to execute all elements of this plan. He added that there was no indication of plotting in the U.S, and said it was unclear whether the alleged plot was linked to the upcoming fifth anniversary of the Sept. 11 strikes.

CBS News correspondent Bob Orr says officials will not comment on who is behind the plot. One official told CBS News: "We know of no immediate threat to U.S. domestic flights. But we are taking this very seriously. We view the plot as serious and substantial."

Deputy Commissioner Paul Stephenson says the arrests were in London, its suburbs and in Birmingham, and that searches continue in a number of locations.

Britain's home secretary, John Reid, says about a dozen of the suspects were "very close" to succeeding in the plot, which he describes as "significant" and designed to cause loss of life. The threat level in England has, for the first time ever, been raised to critical. That means the threat of an attack is imminent, MacVicar reports.

Peter Clarke, head of the Metropolitan Police anti-terrorist branch, said surveillance had been carried out for months before police arrested the 21 people overnight. Sources tell Stewart that several of the 21 suspects picked up so far may not have been actual bombers, but people with material knowledge of the plot.

"We have been looking at meetings, movements, travel, spending and the aspirations of a large group of people," Clarke said. "All the arrests were made on suspicion of the commission, preparation or instigation of acts of terrorism."

British Airways canceled all flights between Heathrow airport and points in Britain, Europe and Libya for all of Thursday. Extra security measures in place at airports leaving Britain include a ban on carry-on luggage, including purses too big to fit in a passenger's pocket. Passengers are only allowed to take personal items — such as passport, eyeglasses and cash — if they are in see-through plastic bags.

A British police official says the suspects are "homegrown," though it was not immediately clear if all of the people in custody are British citizens. The official says police investigating the foiled attack are working closely with the South Asian community.

A U.S. law enforcement official said there have been no arrests in the United States connected to the plot.

A senior U.S. counterterrorism official said authorities believe dozens of people — possibly as many as 50 — were involved in the overseas plot. 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.

"It's disrupting huge commerce, disrupting plans here in the United States and abroad, and they would not have gone with precautions like this unless it was a very serious threat," said Christopher Whitcomb, a terrorism expert for CBS News. "It could have been catastrophic."

Blair's office said in London that the prime minister, vacationing in the Caribbean, briefed President Bush, vacationing at his ranch near Crawford, Texas, on the situation overnight.

"The United States and the United Kingdom are fully united and resolute in this effort and in our ongoing efforts to secure our respective homelands," said Chertoff.

U.S. intelligence, particularly the CIA, has been working closely with Britain on the investigation, which has been ongoing for months, the second official said.

"We cannot assume that this threat has been completely thwarted," Chertoff said Thursday. "There's sufficient uncertainty as to whether the British have scooped up everybody."

As a result of the plot, new security precautions have been put into effect at airports in both England and the United States. Massachusetts Gov. Mitt Romney said he will activate the National Guard to help with security at Logan International Airport for the first time since the Sept. 11 terror attacks. And airlines in South Korea, Singapore, Japan and Australia all imposed new restrictions on carry-on and other luggage for jets in accordance with requests from American and British authorities.

Britain's Transport Secretary Douglas Alexander, speaking at a Thursday news conference in London, asked air travelers to show patience and understanding of the new rules, which he says will be in effect "as long as the situation demands."

Heathrow airport, the busiest in Europe, was closed to most European flights Thursday morning. The block on incoming traffic applies to flights of three hours or less, affecting most of the incoming traffic from Europe, according to an airport spokesman.
Air France canceled all morning flights to Heathrow, saying it acted because the airport is "saturated." The French interior minister cancelled his vacation to return to Paris for an urgent meeting on security with police and intelligence officials.

Among the other tight security measures at airports leaving Britain, eyeglass cases are not permitted. There are also rules on medicines: prescription medicines can be taken on board, in an amount necessary for the duration of the flight, unless they are in liquid form, in which case the authenticity of the medicine must be verified.

Contact lenses are allowed, but bottles of contact lens solution — which were eyed years ago as a potential terrorist tool — are not.

Keys are allowed as carry-ons. Electronic key chains are not, and all electric or battery-powered items, including laptops, cell phones, iPods, must be in checked, not carry-on, baggage.