Police are continuing to question a number of suspects over an alleged plot to bomb planes leaving the UK. All UK airports have now relaxed restrictions on hand luggage.
What do we know about the alleged plot?
Authorities believe the aim was to blow up US and UK aircraft flying from the UK to locations across the US.
Security experts believe that potentially extremely effective liquid explosives would have been used, taken on board in hand luggage, possibly disguised in fizzy drink bottles. Detonators were to be disguised as electronic equipment.
It is thought there could have been three waves of attacks targeting three aircraft each time on different days, with the perpetrators hoping the destruction of the planes would have destroyed evidence of how they carried out the attack allowing them to attack again after a pause.
When would the attacks have been carried out?
This is unknown, but investigators thought the attacks were imminent, planned for not much later than a couple of days after arrests on 10 August.
US homeland security chief Michael Chertoff, who said the plot was "suggestive" of al-Qaeda, claimed the planning was in its "final stages".
Why were the arrests made when they were?
Police and security services decided to act after new intelligence was received early last week. It is not clear exactly what the information was, says BBC security correspondent Gordon Corera, but it made officials realise they did not know the full picture and they became alarmed.
One possibility is that action was taken out of concern the recent arrest of people in Pakistan in relation to the alleged plot may have persuaded co-conspirators in the UK to speed up their plans, or go underground.
What is the focus of the investigation?
Police are continuing to search woods near High Wycombe in Buckinghamshire. King's Wood is near to one of the properties which was searched during Thursday's anti-terror raids.
BBC Home Affairs Correspondent Daniel Sandford also said a handgun and a rifle had been found in properties being searched by police.
On Thursday, 24 people were arrested in Birmingham, High Wycombe and east London after an extensive surveillance operation.
Of those arrested, 23 are still being questioned and one person has been released without charge. A 25th person was also separately arrested and later released without charge.
Sources say all those held are believed to be British nationals, some with links to Pakistan.
The assets of 19 of them have been frozen. One is believed to be a worker at Heathrow.
Unlike the investigation into last year's London bombings, security sources have told the BBC there is a clear connection between the alleged terror plot and senior members of al-Qaeda hiding in Pakistan.
Home Secretary John Reid has said the operation had "an international dimension" and praised Pakistan for the help it had given the investigation.
Officials in Pakistan have arrested seven men in the cities of Lahore and Karachi, of whom two are British nationals of Pakistani origin.
The Pakistan Foreign Ministry has identified one of the men as Rashid Rauf.
Tayib Rauf, believed to be his relative, is among those arrested in the UK.
The seven arrested in Pakistan are said to have provided important information about the suspected UK plot, and the country's foreign ministry has said there are "indications of [an] Afghanistan-based al-Qaeda connection".
Al-Qaeda's chief strategist, Ayman Al-Zawahiri, has made a number of speeches in recent months in which he has singled out Britain for punishment for policies he says are hostile to Muslims.
Al-Qaeda has also had a long fascination with blowing up planes. In 1995 an al-Qaeda plot to destroy 12 US passenger planes as they crossed the Pacific Ocean was intercepted by police in Manila.
How long can police hold the people being questioned about the alleged plot?
The police can question the suspects for a maximum of 28 days from the date of the initial arrest before having to decide whether to charge them. Currently, the Met Police has been granted warrants for the further detention of the suspects until Wednesday.
What level of threat remains?
The Joint Terrorism Analysis Centre (JTAC) has now downgraded the security threat level to the UK to "severe", meaning an attack is "highly likely" but not imminent.
Last week, the level was raised to "critical", the highest rating, and it remained at this level for four days.
NEW THREAT LEVELS
Low - an attack is unlikely
Moderate - an attack is possible but not likely
Substantial - strong possibility of an attack
Severe - an attack is highly likely
Critical - an attack is expected imminently
The lowering of the threat level means the ban on taking hand luggage on to flights from the UK has been lifted, although some restrictions remain.
Announcing the new security level on Monday, the home secretary warned that "the change in the threat level does not mean that the threat has gone away".
Last week, the FBI said they were following "new leads" in their investigation into the suspected terrorist plot uncovered in London, and are working on "a fresh new wave" of material provided by the police and secret services in the UK.
A spokesman told the BBC's Guto Harri Britain had been "superb at sharing information" about the associates, contacts, and movements of those still being questioned by police.
However, Harri says there are also concerns in the US that not everyone suspected of involvement in this plot has been rounded up, and that some of those still at large could be dangerous.
But British sources have told BBC security correspondent Gordon Corera that they do not recognise the idea there are identified people they have been looking for that they have not found.
They do acknowledge it is possible there are people involved who had not been identified as being involved, and therefore could be still be at large.
How long will disruption and increased security measures continue?
All UK airports have now relaxed the restrictions on hand luggage introduced last Thursday.
The new guidelines - which allow one item of hand luggage the size of a laptop computer bag - were delayed until Tuesday at Heathrow and Gatwick.
Most airports have reported business as usual, while 45 flights were cancelled at Heathrow, the worst-affected.
Airport operator BAA has said searches mean delays are likely to continue.
The travel disruption is improving, although some flights are still being cancelled as airports try to clear the backlog.
Restrictions on hand luggage were reduced on Monday but airport operator BAA said the changes were being phased in gradually and urged passengers not to bring hand luggage before Tuesday.