By Adrian Croft
Thursday, August 10, 2006; 2:13 AM
LONDON (Reuters) - British police said on Thursday they have thwarted a plot to blow up aircraft in mid-flight between Britain and the United States, arresting a number of people in the London area.
Britain's security service MI5 raised the threat level to the country to "critical" from "severe," meaning an attack is expected imminently.
News of the arrests and heightened security threat came amid high international tension over Israel's war against Hizbollah in Lebanon.
Police said the aim of the plot was to detonate bombs smuggled on board aircraft in hand luggage.
"A major terrorist plot to allegedly blow up aircraft in mid-flight has been disrupted in a joint, pre-planned, intelligence-led operation by the Metropolitan Police anti-terrorist branch and security services," a London police spokesman said.
He said police believed the intention was particularly to target flights from Britain to the United States.
The BBC said about 18 people were arrested.
The police spokesman could not confirm the figure, saying only that an unspecified number of people had been detained in London during the night. The operation was the culmination of a major covert counter-terrorist operation lasting several months, he added.
Following the arrests, security at all British airports was increased and additional security measures put in place for all flights.
British Prime Minister Tony Blair has come under strong criticism at home and abroad for following the U.S. lead and refusing to call for an immediate ceasefire in the conflict between Israel and Lebanese Hizbollah guerrillas.
The security alert comes 13 months after four Islamist suicide bombers killed 52 people on London's transport network on July 7 last year.
The Home Office Web Site showed the threat rating had been raised to "critical" from "severe" earlier on Thursday. Britain has a five level rating for security threats, ranging from low to critical, which is the highest level.