Some warned, meanwhile, that such security measures are not sustainable.
"It sets a precedent," said Shadi Hamid, an analyst with the Brookings Doha Center. "What happens if you keep on getting credible threats?"
The countries with closure orders covered much of the Muslim and Arab world, from Mauritania in the west to Bangladesh in the east.
The State Department said 19 diplomatic posts will remain closed through Saturday "out of an abundance of caution." State Department spokeswoman Jen Psaki said the decision to keep the embassies and consulates closed is "not an indication of a new threat."
Diplomatic facilities will remain closed through Saturday in Egypt, Jordan, Libya, Saudi Arabia and Kuwait, among other countries.
In recent days, U.S. officials have said they have received significant and detailed intelligence suggesting a possible attack, with some clues pointing to the al-Qaida terror network. The State Department said the potential for terrorism was particularly acute in the Middle East and North Africa, with a possible attack occurring on or coming from the Arabian Peninsula.
"The threat was specific as to how enormous it was going to be and also that certain dates were given," Rep. Pete King, R-N.Y., who chairs a House panel on counterterrorism and intelligence, told ABC on Sunday.
King said he believes al-Qaida "is in many ways stronger than it was before 9/11 because it has mutated and it's spread in dramatically different locations." The terror network's Yemen branch, known as al-Qaida in the Arabian Peninsula, "is the most deadly of all the al-Qaida affiliates," King said.
In Jordan, a counterterrorism official said available information pointed to a potential threat to U.S. interests in the Arabian Peninsula, specifically in Yemen, and that this prompted the temporary closure of U.S. missions across the Muslim world. He spoke on condition of anonymity as he wasn't authorized to discuss the issue with journalists.
In Yemen's capital, Sanaa, security was beefed up Sunday around the U.S. Embassy building and the nearby Sheraton Hotel where U.S. Marines stay.
Police set up a checkpoint at an access road leading to the embassy, asking some drivers for identification before letting them pass. Soldiers typically guard the area around the embassy, but on Sunday they were spread out in a wider radius. Cars were prevented from stopping outside the Sheraton, where two armored vehicles sat out front.
A Yemeni security official said the request for extra security came from Washington. He spoke on condition of anonymity because he was not authorized to release the information.
Extra security also could be seen near U.S. embassies in Bahrain, Iraq and Jordan.
In the Iraqi capital of Baghdad, troops set up new blast barriers last week to block several streets leading into the city's already heavily fortified Green Zone, home to the sprawling U.S. Embassy and Iraqi government offices. Troops also intensified searches of those entering the Green Zone, opening car trunks and frisking male passengers.
In the Jordanian capital of Amman, a Jordanian security officer said bomb squads searched the perimeter of the U.S. Embassy while additional security vehicles were deployed in the area, including troop carriers with special forces trained in counterterrorism. Security also was tightened around the homes of U.S. diplomats in Amman, said the officer, who spoke on condition of anonymity in line with regulations.
The State Department, meanwhile, urged U.S. travelers to take extra precautions. Expats appeared to take the warnings in stride.
Rebecca Proctor, a magazine editor in Dubai, said she had heard of the threats, but wasn't going to change her routine. She said she spent the day in meetings around the city, despite warnings from a friend not to visit tourist areas or speak English in public.
"I'm not going to stay inside and huddled up," said Proctor, who is from New London, Connecticut.
In Amman, San Francisco native Wendy LeBlanc, an education consultant, also said she wasn't changing her routine.
"Right now, the biggest threat here is a stray bullet from celebratory gunfire," said LeBlanc, referring to the custom in parts of the Arab world to shoot in the air to mark important occasions.
The decision to close the U.S. diplomatic missions on Sunday a work day in most of the region came almost a year after an attack on a U.S. diplomatic post in Benghazi, Libya, that killed the ambassador and three other Americans.
Some argued that heightened security measures could give al-Qaida an inadvertent image boost.
"The closure of some U.S. embassies sends a wrong message to the world that al-Qaida is still strong," said Qais Mohammed, an engineer from Baghdad. "I think that adopting balanced and fair policies toward the Arab and Islamic world is the best way to safeguard U.S. embassies and interests in the region."
Laub reported from the West Bank. Associated Press writers Jamal Halaby and Dale Gavlak in Amman, Jordan, Sameer N. Yacoub in Baghdad, Michael Casey in Dubai, Aya Batrawy in Cairo and Max J. Rosenthal in Jerusalem contributed to this report.