All of Scotland's airports are on a high-security alert after an operation to foil an alleged plot to blow up planes from the UK in mid-flight.
A heavy police presence was evident at Glasgow Airport
It is thought the plan was to detonate explosive devices smuggled on to aircraft in hand luggage. Scotland Yard has arrested 24 people.
Stringent extra security measures are in place at Glasgow, Edinburgh, Aberdeen, Inverness and Prestwick.
Security at all UK airports is now at "critical", the highest level there is.
This is the first time it has been raised to such a state.
During the night, police made arrests in London, High Wycombe and Birmingham after a counter-terrorist operation they said had lasted several months.
Only the barest essentials - including passports and wallets - have been allowed to be carried on board in transparent plastic bags. Everything else must be put in hold luggage and checked in.
Chief Constable Ian Latimer, president of the Association of Chief Police Officers in Scotland, urged the public to be "alert but not to be alarmed".
No extra charge
He added: "There is no intelligence of any direct link with today's incidents to Scotland."
Virgin Trains said it would accept domestic air tickets on its trains for services to Scotland from regional destinations.
Stations include Manchester, Birmingham, Bristol and Reading going to and from Edinburgh and Glasgow, but excluding London stations.
Sandy MacPherson of the Scottish Passenger Agents' Association said that people whose flights had been cancelled on Thursday should expect to be rebooked by their airline at no extra charge.
He added: "I should think they'd get a refund if they can't travel at all.
"The same will apply to people who were due to be flying out of Scotland through London and onto another destination, if they were flying all the way through on a through ticket.
"On the other hand if they had booked a separate ticket to fly from a Scottish airport to London, they are at the mercy of the airline that is flying out of London.
"They are not obliged to refund the ticket, it will be a matter of whether the airline is being sympathetic."
A spokesman for BAA Scotland, which operates Glasgow, Edinburgh and Aberdeen airports, said that additional staff had been brought in to cope with the increased security checks.
More than 70 flights were cancelled - 30 at Glasgow, 25 at Edinburgh and 18 at Aberdeen - and many more departures were delayed.
At Aberdeen Airport, oil industry worker Gregor Tait, 33, was due to fly to Heathrow with his wife, their two young children and his brother for a wedding in Essex, but the flight was cancelled.
They decided, rather than wait for a later flight, they would make the journey by car.
Passengers are being given plastic bags to carry some possessions
Mr Tait said: "It's a hassle, but I absolutely understand. I would rather this than the alternative."
At privately-owned Prestwick Airport in Ayrshire, a summary of the Department of Transport's additional security measures is being distributed to passengers.
Richard Jeffery, managing director of Edinburgh Airport, said extra staff had been called in.
Highlands and Islands Airports said the heightened security was being applied at Inverness and its other nine airports across the north of Scotland and the Western Isles.
Scotland's smallest airports have not escaped the alert. Flights in and out of Orkney and Shetland have been delayed and police officers are on duty at both Kirkwall and Sumburgh airports.
Extra security measures have been put in place at the Scottish Parliament where all bags and cars are now being subjected to screening and searches.
CBI Scotland director, Iain McMillan, said that despite the disruption and inconvenience to business and tourism the top priority must be public safety.
He said: "Hopefully the disruption will prove to have only a short-term impact."