Americans awoke Wednesday to a heightened state of alert after the Department of Homeland Security (search) raised the terror level to orange -- or "high" -- amid concerns that recent bombings overseas and intercepted chatter are signs of impending Al Qaeda (search) attacks.

Meanwhile, the pan-Arab satellite station Al-Jazeera aired excerpts Wednesday of what it said was an audiotape on which Usama bin Laden's top lieutenant called on Muslims to carry out attacks similar to those of Sept. 11, 2001.

"Consider your 19 brothers who attacked America in Washington and New York with their planes as an example," a strong voice could be heard saying as the station showed a file photo of the lieutenant, Ayman al-Zawahri (search), wearing a white turban.

In Saudi Arabia, three Moroccans were arrested Monday in Jiddah as they were trying to hijack a civilian airliner, a Saudi security source said Wednesday.

Saudi officials said they believe the Moroccans, who had tickets to board a flight to Khartoum, Sudan, planned to hijack the plane and crash it into a high-rise building in Jiddah, replicating the Sept. 11 attacks.

The national alert level was raised in the U.S. as general intelligence pointed to a domestic attack. But there were no credible details on a time, location, method or target. Law enforcement pointed to last week's bombings in Saudi Arabia and Morocco as part of a coordinated series that could spread.

"The U.S. intelligence community believes that Al Qaeda has entered an operational period worldwide, and this may include attacks in the United States," said Homeland Security Secretary Tom Ridge.

Ridge warned of attacks similar to those in Riyadh, Saudi Arabia, last week where homicide bombers detonated truck bombs at several lightly defended residential complexes.

Officials told Fox News that the decision to raise the alert was based in part on Al Qaeda-affiliated e-mails and Web sites, several of which cited several major U.S. cities as targets.

A U.S. official said the Internet "chatter" focused on the Arabian Peninsula, southeast Asia and east Africa, but there were indications the next possible attacks may be in Europe and the United States.

At least one Web site urged Muslims "to get out of New York, D.C. and Boston" by Wednesday for what is believed to be a large act of terrorism, officials said.

But the reliability and imminent danger of the threats were not confirmed.

FBI Director Robert Mueller also stressed Wednesday that there is "no specificity" as to potential targets or times.

"The American public ought to go about its business understanding that it ought to be vigilant," he said on ABC's Good Morning America, saying the system of alerts deters attacks. "We know that from our interrogations" with suspected terrorists, Mueller said.

But Mueller said Al Qaeda is "on the run" and that authorities believe that bin Laden "is still out there. I think there's some concern about what kind of control he has over the people."

Most Americans will carry on with life as usual, experts said.

"I think Americans are wise to this and there's a lot of common sense in this country … you have to do what you have to do," former presidential adviser David Gergen told Fox News Wednesday. "Terrorists are not going to scare us."

California police began working 12-hour shifts. National Guard troops in New York were manning subways and bridges. In Washington, the Capitol police SWAT team conducted random patrols.

At the Pentagon Wednesday, military police who normally wear berets donned helmets and bulky flak vests to guard some entrances of the sprawling military headquarters.

The building's security force was briefed by a terrorist specialist in the morning, reminding them to be extra vigilant.

LA police chief William Bratton said authorities there have boosted patrols, but said local governments badly need financial assistance.

"We've been promised close to $30 million and we've filled out a few applications but haven't seen it yet," he said on CBS' The Early Show.

"Boy, it's sort of like molasses," Bratton said.

Massachusetts Gov. Mitt Romney said additional police officers will be dispatched statewide to protect key state assets. In Boston, extra police have been assigned to Fenway Park, where the Red Sox are playing the Yankees this week.

Tuesday's decision to raise the alert to orange, signifying a "high" risk of attacks, from yellow, meaning "elevated," came after President Bush's homeland security council met Tuesday. Orange is the second-highest on the five-color alert scale.

The intelligence picked up recently included two electronic transmissions that discussed the possibility of an attack on New York, Washington, Boston and U.S. coastlines. But law enforcement officials doubt just how valid those threats were and stressed that they were not the main reason the threat level was raised.

Ridge encouraged governors and mayors to deploy extra police and take other precautions, particularly at large public gatherings during the upcoming Memorial Day weekend.

The United States, Britain and Germany temporarily closed their embassies and consulates in Saudi Arabia on Tuesday.

The Federal Aviation Administration will temporarily prohibit flights over sports stadiums and restrict small private planes flying within a 17.25-mile radius of the Washington Monument, said agency spokeswoman Laura Brown.

The FAA will also require private planes flying into three Washington-area airports -- College Park, Potomac and Washington Executive/Hyde -- to first land at Tipton Airport in Maryland so the pilots can be checked.

A Saudi official said dozens of Muslim militants linked to Al Qaeda were believed ready to volunteer for homicide bombings like the ones in Riyadh.

American counterterrorism officials suspect that Al Qaeda leaders in Iran are directing the attacks. Iran denies sheltering the terror leaders.

The Bush administration has raised the terror alert level from yellow to orange three times previously. Each time, the level was lowered back to yellow after a few weeks. The lowest two levels, green and blue, and the highest, red, haven't been used.

During previous alerts, no domestic attacks were apparently attempted. Some question whether the increased alerts do anything more than frighten the public and cost taxpayer dollars, but the administration says they deter attacks.

"Raising the alert level is prudent when the circumstances warrant doing that, but each time it means direct costs to state and local governments," said Sen. Patrick Leahy, D-Vt. "We are continually tapping the coffers of local police, fire and rescue units to counter these threats, and many of them are being tapped out."

The FBI, in bulletins sent Friday and Tuesday to U.S. law enforcement, said the attacks in Saudi Arabia and Morocco could be "a prelude to an attack on the United States."

State and local police were urged to remain vigilant for signs of surveillance against potential targets or reports of attempts by anyone to obtain explosives. The FBI said any future attack would look similar to recent ones overseas.

Defense Secretary Donald H. Rumsfeld said U.S. anti-terrorism efforts have hurt Al Qaeda, but not killed it.

"The fact that you're having some success and you're capturing some money and you're capturing some people and you're making life more difficult for them does not mean that they're gone. They're not," Rumsfeld said.

Fox News' Bret Baier, Rita Cosby and the Associated Press contributed to this report.