The announcements follow what U.S. officials said was an assessment of information that al-Qaida appeared to be plotting attacks on European cities.
The American alert did not identify any particular countries and did not urge Americans to avoid particular venues. It was a step below a formal "travel warning," a designation telling U.S. citizens to avoid traveling to particular places.
U.S. officials emphasized that Americans should not alter any travel plans because of the alert. "We are not, repeat not, advising Americans not to go to Europe," said Patrick Kennedy, the undersecretary of state for management. "We're not saying don't visit major tourist attractions or historic sites or monuments." He suggested that U.S. citizens register with embassies abroad, avoid civil disturbances and not discuss travel plans with others.
European authorities fell in line with the United States in urging increased caution. They have been warning for the past three weeks that the danger of a terrorist attack in Europe has been higher than usual, but they issued no specific information on what new intelligence has led them to ratchet up the alert level.
"We are in contact with our American partners, and we are watching the situation," said a statement in Brussels from Cecilia Malmstrom, the European Union's commissioner for domestic affairs.
In its announcement, the State Department said, "Current information suggests that al-Qaida and affiliated organizations continue to plan terrorist attacks." It also said some European governments were warning of "heightened threat conditions."
"U.S. citizens are reminded of the potential for terrorists to attack public transportation systems and other tourist infrastructure," the notice said. "Terrorists have targeted and attacked subway and rail systems, as well as aviation and maritime services. U.S. citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when traveling."
The State Department alert for Europe is less specific than one the agency issued for India last month connected to the Commonwealth Games in New Delhi. That alert said that "U.S. citizens are advised to monitor local news reports and consider the level of security present when visiting public places, including religious sites."
State Department alerts and warnings are not uncommon - more serious warnings are posted now for 31 nations, including tourist destinations such as Mexico, Israel, Nepal and Kenya. But highlighting threats across an entire continent, and especially Europe, is far less common and could affect tourism and study-abroad programs.
In addition, thousands of U.S. troops based in Germany were placed under a curfew Friday night and were told not to wear their uniforms off base, according to an order obtained by CNN.
Although the State Department issued an alert about Europe rather than a warning, the announcement nonetheless attracted wide attention. A White House spokesman e-mailed a statement describing President Barack Obama's role in combating the alleged threat.
"From the day we became aware of this latest plot, the President made clear we need to do everything possible to disrupt this plot and protect the American people," spokesman Nicholas Shapiro wrote. "Whether the State Department issues a travel alert is the decision of the State Department, but this alert is responsive to the President's direction that we spare no effort."
The European embrace of the heightened alert would seem to deflect suggestions that the effort to highlight Obama's role, four weeks before midterm elections, was politically motivated.
During George W. Bush's presidency, Democrats routinely charged that threat levels appeared to be raised in advance of elections. Former Homeland Security secretary Tom Ridge seemed to confirm that political considerations played a role when he wrote in a memoir last year that he had to battle an effort by senior Bush administration officials to raise the nation's terrorism alert level in the days before the 2004 presidential vote. Ridge also disclosed that the White House added language praising the president to an August 2004 statement on a newly discovered terrorist threat. Ridge said he regretted his failure to block the new wording.
Malmstrom, who received a phone call from Homeland Security Secretary Janet Napolitano, asked for more details on intelligence underlying the U.S. warning, according to diplomatic sources speaking anonymously to reporters in the Belgian capital. EU interior ministers, who are in charge of security, have planned a meeting Thursday in Luxembourg to share information on the situation, they said.
Britain's Foreign Office issued its own warning to British citizens who might be traveling to Germany and France. "Like other big European countries, they face a strong terrorist threat," a spokesman said.
France's interior minister, Brice Hortefeux, said French intelligence services were in close contact with their U.S. counterparts and remained on a high-level alert that has been in effect since early last month.
"Naturally, we are vigilant, and we take into account what our American ally might say," Hortefeux told reporters.