Security at Wales' airports has been stepped up following the disruption of a plot to blow up planes in mid-flight from the UK to the US.
At Cardiff International Airport, extra staff have been drafted in as part of the heightened security measures.
More than 9,000 passengers were due at Cardiff. Two flights to Orlando in the US have taken off after long delays.
All passengers are facing delays and are being told to allow extra time and to follow hand luggage instructions.
Police have made a number of arrests in the London area and the West Midlands after an anti-terrorist operation lasting several months.
Security at all airports in the UK has been tightened. MI5 has raised the UK threat level to critical - the highest possible.
Superintendent Sue Hayes, of South Wales Police, said the force was on "high alert".
"There is no specific intelligence to suggest that Cardiff airport in particular is under any threat or a target for terrorists. However, we are responding to the national situation," she added.
Five arrests were made for alcohol related public order offences at 1300 BST, she confirmed.
CARDIFF AIRPORT AT A GLANCE
Outbound flights delayed by at least two hours
Passengers hand searched
Vehicle access limited
In-bound evening flights from Belfast Int, Edinburgh, Glasgow and one from Amsterdam cancelled
Outgoing flights to same four airports cancelled
Jon Horne, managing director of Cardiff International Airport, said flights were being delayed by an average of two hours, but three to four hours in some cases.
The last scheduled flight on Thursday is due out at 2030 BST to Dublin.
Mr Horne said: "It's a very dynamic situation, we are just watching to see how things are affected.
The airport is advising that no hand luggage is allowed on any flight with limited items allowed to be taken on board in a transparent plastic bag.
Items being carried by passengers will be X-ray screened and passengers' footwear will be examined.
Passengers arriving at the airport to fly out on holiday were philosophical.
Joanne Redfern, from Plymouth, was on her way to Turkey for two weeks, said they had been watching the news before they left home.
"I've had worried phone calls from my mother but I'm not worried. It will be even more safer than usual to fly.
"We've packed our hand luggage items into freezer bags before we left the house after we heard the warnings on the news.
Maggie Sibbald, from Swansea, who was heading to Palma: "I think if I'd had a seven o'clock flight this morning I'd be a lot more worried than I am because everyone has already heard what's going on and is prepared.
"It wouldn't have been like that this morning, it would have been chaos."
Friends Tom Cornelius and Steve Maunder from Swansea were flying out to Spain. "When we first heard about our first thought was that it was going to spoil our holiday.
"But this afternoon it seems to be OK. It hasn't affected out plans at all so far.
"I think it's going to be one of the safest places in the world because of all the safety checks going on."
South Wales Police said officers are continuously liaising with the Metropolitan Police, Special Branch and other emergency services.
"Our policing operations are in response to the risk which is currently evident across the whole of the UK, and is no greater or lesser in Wales than anywhere else. People shouldn't be alarmed, but be alert, and stay vigilant," a spokesman said.
Police said the security measures are also affecting Swansea Airport and Swansea Ferry Port.