Five people were being questioned by police today after being arrested under anti-terror laws near airports.
Scotland Yard detectives were questioning a 37-year-old man found with a live grenade in his luggage at Gatwick yesterday.
The man, who got off a British Airways flight from Bogota in Columbia, was travelling on a Venezuelan passport and police are almost certain he is from that country.
Checks are being made in Caracas to establish his background while he remains at high security Paddington Green police station in London.
In a separate operation, four people were held near Heathrow by Thames Valley Police.
They were arrested in Langley, Berkshire, where officers have set up road blocks and have been routinely stopping and checking cars and vans.
The arrests were made yesterday but details emerged only today and were described as “precautionary“.
They were not being linked to a specific plot against the airport. The four are being questioned at a nearby police station.
There were two other arrests at Hounslow, near Heathrow, yesterday when two men were held by police carrying out random stops of cars.
The arrests were also described as “precautionary” and were not being linked to an alleged al Qaida plot to launch a missile against a low-flying aircraft at Heathrow.
Today the two men were released with no further action. One was handed over to the Immigration Service.
In a seperate scare, part of Terminal 2 at Heathrow was evacuated today while a suspect bag was investigated, causing disruption to passengers.
It was later declared a false alarm.
At Stansted the access road to the terminal was closed for five hours today amid heightened security measures.
It is understood the road was closed due to the arrival and departure of flights by the Israeli airline El Al to Tel Aviv, which are considered a potential target for terrorists.
The road was guarded by armed police.
Scotland Yard Deputy Assistant Commissioner Andy Trotter said the Metropolitan Police were doing all they could to protect the capital.
“We’re taking every measure necessary to keep London safe and the evidence of our activity is out there on the streets, and we’ll do everything that is required to deal with the threat that we perceive at the moment,” he said.
The grenade found at Gatwick yesterday was on flight BA 2048 carrying 125 passengers from Bogota in Colombia at 1.23pm.
Passengers at Gatwick were immediately evacuated from the airport’s North Terminal after the grenade find and outbound flights were suspended. The terminal remained closed for several hours until it was reopened at 7.45pm last night.
Around 150 people were forced to spend the night wrapped in blankets.
A BA spokeswoman said today that around 4,000 of its passengers were affected by the security alert yesterday.
More than 400 soldiers and around 1,700 extra police officers were drafted in to Heathrow and other key London locations on Tuesday and security was also beefed up at Manchester airport.
Police and Government ministers say the move was the result of “specific intelligence” about a terrorist threat.
It included the possibility that al Qaida terrorists have smuggled a shoulder-held rocket launcher into the UK and may try to shoot down a plane.
Terrorism experts said surface-to-air missiles that went missing 20 years ago in Afghanistan could be behind the threat.
At Heathrow many passengers refused to be intimidated by the terror threat.
Stephanie Nelson, 26, from Chicago, who had arrived for a short holiday in London, said: “It’s scary but I’m not going to let it stop me from flying.
“Life has to go on. The police are doing everything they can to stop another incident like September 11 happening.”
But Trevor Matthias, 19, a musician from Chicago who was touring the UK with his band, said: “I was really scared. I didn’t want to fly with all this terrorist stuff going on. I didn’t want to come at all.”