Fighter aircraft are patrolling the skies and anti-aircraft systems have been deployed while key infrastructure elements such as banks and transport services have been put on high alert.
US military bases and diplomatic missions worldwide have also boosted their security and two embassies and a consulate in south-east Asia have been temporarily closed.
President George W Bush has issued a message of thanks to people around the world who have supported the US in what he called the struggle between "civilisation and ruthless violence".
New York:
0800 (1200GMT) Memorial service at Ground Zero
0846 (1246GMT) One minute silence
1740 (2140GMT)President Bush lays wreath at Ground Zero
Washington:
0835 (1235GMT) Memorial ceremony at the White House
0930 (1330GMT) Memorial ceremony at the Pentagon
1000 (1400GMT)President Bush speaks
Pennsylvania:
1245 (1645GMT) President Bush lays a wreath at Shanksville, where one hijacked plane crashed
The government has now put the country on a 'Code Orange' alert, indicating there is a high danger of US interests being subjected to terror attacks.
Earlier, a State Department communique warned US citizens worldwide to be vigilant against a continued threat of terrorist actions.
"The US Government has continued to receive credible indications that extremist groups and individuals are planning additional terrorist actions against US interests," the communique said.
"Such actions may be imminent and include suicide operations. We remind American citizens to remain vigilant with regard to their personal security and to exercise caution."
In the past few days, US intelligence agencies have detected what officials now describe as a marked increase in terrorist "chatter" - information from monitored communications and other sources that are used as a barometer of the likelihood of potential attacks.
American alert
Anti-aircraft missile systems have been deployed at key sites around Washington although the Pentagon says it is only an exercise with no live weapons involved.
However, the missile systems could be equipped with live weapons in an emergency.
BBC Pentagon correspondent Nick Childs says the timing is clearly more than a coincidence - it is a measure of how the US military now treats the threat from the air over America's own soil.
The Federal Bureau of Investigation has told police, utilities, banks and transport services to be on high alert.
White House press secretary Ari Fleischer said the threat of new attacks remained a concern.
"Just given the fact that it's a one-year anniversary, we're going to be on our toes," he said.
President Bush thanked those abroad who had shared America's grief over 11 September and were backing its resolve to "lift the dark cloud of terror" from the world.
The US leader looked ahead to the defeat of terrorism and, without being specific, said he was confident that "old and serious disputes" could be resolved peacefully.
He is due to mark Wednesday's anniversary with a speech to the nation, followed by an address on Thursday to the United Nations General Assembly.
Embassies shut down
US embassies in two mainly Muslim countries in South East Asia - Indonesia and Malaysia - have closed down for the anniversary.
In Indonesia, both the embassy in Jakarta and the consulate in Surabaya were shut down.
Without giving details, US officials said information about "credible and specific threats" against the premises had been received.
The US Navy in Bahrain has issued a warning to shipping in the Gulf of reported plans by al-Qaeda Islamic militants to target oil tankers.
A navy spokesman quoted by Reuters news agency said it was the first warning issued by the navy specifically for the Gulf since 11 September.
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