The US Government has raised the nation's terror alert to its second-highest level.
New York remains a high profile target
Secretary of Homeland Security Tom Ridge said the move followed a substantial increase in intelligence reports about possible threats.
Reports indicated that militants wanted to carry out attacks to exceed or rival those of 11 September, Mr Ridge said.
In New York, Mayor Michael Bloomberg said more police patrols would be now deployed at key sites in the city.
"We are stepping up our counterterrorism operations throughout the city... we always assume we will be a target," Mr Bloomberg said.
At a news conference in Washington, Mr Ridge warned that aircraft could be used - as they were in 2001 - to carry out attacks on the American soil.
"We continue to hear... the interest in using aircraft as a means of attack," he said.
Intelligence intercepts suggested that the threat now from al-Qaeda was greater than at any time since the attacks on New York and Washington, he added.
The tone of his statement was strong enough to signal this was not just a standard warning, the BBC's David Bamford in Washington says.
It reflected the authorities' real concern that something serious may be brewing.
But amid the warnings, Mr Ridge sought to reassure people, saying border security was being stepped up and airports were on full alert.
"Your government is at the ready 24 hours a day, seven days a week to stop terrorist attacks in the holiday season and beyond," Mr Ridge said.
He urged Americans to be vigilant, but to continue with their holiday plans and to enjoy the "spirit of the season".
"America is a country that will not be bent by terror, will not be broken by fear," he said.
His announcement means the national threat level is increased from yellow to orange.
The five-level colour-coded alert system was introduced after the 11 September attacks.
It has never been at the top red alert.
Speaking earlier on Sunday, General Richard Myers, chairman of the Joint Chiefs of Staff, told US television officials were trying to determine whether the increase in electronic "chatter" being picked up was serious.
PREVIOUS ORANGE ALERTS
20 May 2003: after Saudi, Morocco bombings
17 Mar 2003: run-up to invasion of Iraq
7 Feb 2003: intelligence on increased al-Qaeda threat
10 Sep 2002: on first anniversary of 9/11 attacks
"There is no doubt, from all the intelligence we pick up from al-Qaeda, that they want to do way with our way of life," General Myers told Fox news.
Last Friday, the Arabic satellite channel al-Jazeera broadcast a tape said to be of Osama Bin Laden's deputy, Ayman al-Zawahri, in which he vowed to defeat the US and warned that al-Qaeda was pursuing Americans "everywhere".
Has the raising of the terror alert led to you changing your plans?
What good does it actually do releasing these vague alerts? It seems to me the only outcome is that it keeps the level of fear just at the right amount to suit the US government - wait a month or two and just when everyone forgets about it all, out comes another alert. The phrase "Your government is at the ready 24 hours a day, seven days a week to stop terrorist attacks in the holiday season and beyond," says it all for me. When nothing actually happens, the US government can take all the credit even though there isn't anything to actually take credit for other than perpetuating the sense of paranoia.
Ian Ryder, London
I am taking my Husband to New York in a few weeks for his birthday and nothing will make me change my plans. I will not be governed by anyone as to how I live my life. Live life to the full is my motto.
Maria Christofides, Cambridge, United Kingdom
No it has not. I can't help but find it silly that a government can raise its "Fear Level" just to scare its population into forfeiting even more of their civil liberties, since that is what is really happening at the moment. It is truly not the American people's fault that they are up against such a formidable propaganda machine as their government - I just hope the world comes to its senses before it is too late.
Martin Keyser, Johannesburg South Africa
Going about one's life is a bit like Russian Roulette. But how can you stop living? The sooner we catch up with Osama bin Laden and the sooner he is exposed a la Saddam, the better.
Mike Beeson, Manchester, UK