BOSTON -- Gov. Mitt Romney activated the Massachusetts National Guard to assist with security at Logan International Airport after police in Great Britain broke up an alleged terror plot that was intended to cause "mass murder on an unimaginable scale."

Heightened security measures were put in place at Logan after British authorities said that the plan was to blow up planes headed to the United States in mid-flight using explosives smuggled in hand luggage. British Police said they have 24 suspects in custody.

It's the first time since the Sept. 11 2001, terrorist attacks that the National Guard has been called to help at Logan.

"The measures that are being taken relate to visible deterrence as well as detection of visible devices that can be harmful," Romney said.

Romney said the National Guard would help with additional security screening stations being put in place at terminal gates inside the airport.

In Washington, two officials said the terrorists planned to target United, American and Continental airlines. Romney said that the "presumed targets" were not Logan-bound flights.

Officials in London said that extra security arrangements will be in place for flights headed to the U.S.

"The police acting with the security service MI-5 are investigating an alleged plot to bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life," Britain's Home Secretary John Reid said.

The nation's threat level was raised to "severe" or red for all commercial flights originating in Great Britain and bound for the U.S. Within the U.S., the threat level is "high" or orange, and it applies to all commercial planes operating in or destined for the U.S.

"We believe that these arrests have significantly disrupted the threat, but we can not be sure that the threat has been entirely eliminated or the plot completely thwarted," U.S. Homeland Security Secretary Michael Chertoff said.

Passengers arriving at Logan on Thursday experienced long delays in the terminals. The delays were a result of the security screeners spending extra time looking for items now banned from planes.

Passengers and crew are not allowed to bring any liquids on board any flight. TSA officials said items must be in checked baggage, including all beverages, shampoo, sun tan lotion, creams, toothpaste and hair gel. Baby formula and medicines must be presented for inspection at the checkpoint.

"For those people who are going overseas and plan to come back from London, they should recognize that while they can leave the U.S. with a carry-on bag, when they return from London Heathrow, they can have no carry-on bags," Romney said.

Early-morning travelers reacted quickly to the new threat.

"I am a little nervous, but we feel good that they found those guys before anything could have happened. It's a little scary," one traveler said.

"Passengers should expect generally the same experience in traveling. This morning, no doubt, there were some delays as we were evolving the process. I think as time goes on, you'll see a much smoother operation," Federal Security Director George Naccara said.