Armed police are patrolling Bristol International Airport after a terror plot to blow up planes in mid-flight from the UK to the US was foiled.
Severe delays on all flights are expected to last several days
Anti-terrorism forces say the plan to smuggle explosives on board in hand luggage would commit "mass murder on an unimaginable scale".
Passengers have been told they can only take essential items with them into the cabin in clear plastic bags.
Flights to Amsterdam, Nice, Barcelona, and Newcastle have been cancelled.
Armed officers with dogs are conducting high profile patrols at the airport and severe delays are expected to last several days.
Assistant Chief Constable Rod Hansen said: "The safety of the entire airport community is our paramount concern at this time, and we are working together with the airport to ensure that everything is being done to maintain their safety.
"This will inevitably cause delays and disruption. I would ask people to be calm and patient, and understand that everything that is now being done is as a precautionary step with public safety as our foremost priority."
People who are not intending to travel will not be allowed into the terminal.
"To date, 95% of passengers from Bristol International Airport have departed with minimal delays this morning and only five flights have been cancelled from the Airport," said Andrew Skipp, Bristol International's managing director.
"We expect 15,000 people to travel through the airport today, with this week being one of the busiest periods of travel of the year as we are in the midst of the summer holiday season."
Passengers have been advised to arrive earlier than normal for their flights so that all necessary security checks can be made.