The UK's main political parties are "completely united" over a terror plot which would have caused "unprecedented" carnage, the home secretary has said.
"The loss of life to innocent civilians would have been on an unprecedented scale," said Mr Reid.
The home secretary has chaired sessions of the government's emergency committee after 24 terror suspects were arrested.
Tony Blair, who is away on holiday, praised the "immense effort" made by the police and security services.
Downing Street says Mr Blair, who is in the Caribbean with his family, has been in constant touch but there are no suggestions the prime minister plans to return from holiday.
In a statement issued by No 10, Mr Blair said the police and security services had "tracked the situation" for a long time.
"I thank them for the great job they are doing in protecting our country," he said.
"There has been an enormous amount of co-operation with the US authorities which has been of great value and underlines the threat we face and our determination to counter it."
President George Bush thanked the UK government for its "good work in busting this plot".
The UK's terror threat has been raised to "critical", meaning MI5 think an attack should be expected imminently.
And since late on Wednesday night, Mr Reid has chaired two of the meetings of government emergencies committee Cobra, which met again on Thursday afternoon.
The top civil servant at the Home Office, Sir David Normington, chaired another overnight meeting of the committee, which brings together ministers, officials, police and intelligence chiefs.
Deputy Prime Minister John Prescott is in charge of the day-to-day running of the country.
He did not attend the first Cobra meetings but he was at the afternoon meeting.
Government officials said the home secretary would have chaired the committee even if Mr Blair had not been abroad.
'Struggle against evil'
At a news conference in Westminster, Mr Reid said the police and MI5 had decided to make the arrests, with the full knowledge of the prime minister, his deputy, the transport secretary and himself.
He said he had spoken to Conservative leader David Cameron and Liberal Democrat leader Sir Menzies Campbell, who had offered their full support.
"At this important time we have a completely united body politic, which I think is important."
Mr Reid said the UK was engaged in a "long, wide and deep struggle against very evil people".
"This is not a case of one civilisation against another, one religion against another," he said.
"It is a case, in general terms, of terrorists who want to use evil methods against the rest."
He added there was a common cause "among all the people in this country from whatever background, religion or ethnic dimension".
The home secretary refused to say whether those arrested were British-born.
But he said Mr Prescott was contacting MPs and groups from those communities affected.
David Davis, the shadow home secretary, said he would "applaud" the government's approach, saying precautions had to be taken.
"Normally you will find me very sceptical of the government and some of its actions on terror but here I don't think there's any argument," he told BBC Radio 4's The World at One.
"Clearly there's something under way here."
Labour MP Mohammed Sarwar says it is now "imperative" to recall MPs from their long summer break.
But Mr Davis said the terror news was not a reason for Parliament or Mr Blair to return from holiday.
"We should not be jumping through hoops every time a terrorist plot is foiled," said Mr Davis.
Liberal Democrat president Simon Hughes said the government was right to take all necessary precautions.
"It is always better to be more careful than to take risks," he said.
Safe to fly?
Police say they have arrested the "main players" in an alleged plot to blow up planes in mid-flight from the UK to the US.
Security at all airports in the UK has been tightened and delays are expected.
Transport Secretary Douglas Alexander said extra security meant people could feel safe flying.
A source close to Mr Alexander told the BBC the current situation was likely to lead to "lasting changes" to the security arrangements at British airports.