Air passengers and rail commuters are to bear the brunt of heightened security following the three attempted bombings in London and Glasgow Airport.
Patrols of airport terminals have been increased
Cars, including taxis, are being banned from approaching terminals at many airports, and vehicle searches at railway stations are being increased.
Passengers are being urged to travel to airports on public transport or allow extra time if arriving by car.
Police said the increased presence would act as a "visible deterrent".
The UK moved to its highest level of terror alert - critical - after a Jeep Cherokee crashed into the doors of Glasgow Airport's main terminal and burst into flames on Saturday afternoon.
The incident came a day after two cars loaded with petrol, gas cylinders and nails were found in London's West End.
'Armed response patrols'
Security has been increased across the country, with travellers expected to experience the worst disruption.
Scotland Yard said there would be a heightened police presence in the capital at mainline stations and on the streets to "act as a highly visible reassurance".
Armed response vehicles will also be patrolling major transport hubs and officers will be using stop and search powers.
A spokesman said: "As a visible deterrent and disruptive tactic, officers will continue to use their powers of stop and search under the Terrorism Act."
Today's travel situation:
- Traffic at Heathrow Airport "busy but moving" and no flight delaysGlasgow Airport back to normal but delays and cancellations possible
- Congestion at Gatwick Airport from police checks on approach roads
- Stansted, Liverpool John Lennon, Southampton, Edinburgh and Aberdeen airports all reporting normal service
Metropolitan Police Assistant Commissioner Tarique Ghaffur praised the public for "showing such resilience during this challenging time for London".
He said it was important for people to carry on with their normal daily lives while remaining alert and vigilant.
He added: "I'd like to reassure the public that we're doing everything possible to make them safe by putting out extra patrols and deploying other protective measures. Safety and security is our number one priority."
A number of airports have already taken extra security measures, including Heathrow, Edinburgh, Newcastle, Birmingham, Manchester and Blackpool.
BAA said access to forecourts and drop-off zones at terminals across the country would be "severely restricted".
A spokesman said people who usually drive up to entrances were being advised to use public transport or to use short-stay car parks.
John Morris, head of corporate affairs at Birmingham Airport, told BBC News how the increased security was affecting the airport.
"We are geared up to do this indefinitely, people recognise that we'll do this as long as we need to," he said.
"People are listening to instructions and doing what we want, so there are no flight delays at all."
The US is increasing the number of air marshals on flights between the US and UK in response to the failed attacks.
British Transport Police said police patrols were being stepped up across the rail network and random searches on vehicles approaching stations and passengers were being increased.
A spokesman added: "We continue to work closely with our colleagues from Home Office forces to ensure high visibility patrols in and around stations.
"Passengers and station staff have been briefed to be extra vigilant and report any suspicious activity immediately."
Security is also being increased at the Wimbledon Championships, in south-west London, which are resuming after a day's break.
Road blocks have been set up in front of the gates to the All England Club. Police said tennis fans were being advised to use public transport rather than cars, unless they had already obtained accredited parking.