A spate of anti-American protests including several firebombings were reported around the world yesterday, but the State Department said there were no injuries, only minor damage and no "specific and credible" threats of terrorism.

In what appeared to be the most serious incident following the U.S.-led attack on Iraq, a powerful bomb exploded in a busy New Delhi marketplace, just outside the offices of an American Airlines travel agency. Police said no one asserted responsibility for the attack.

In England, police rounded up 35 Iraqi nationals as suspected agents of Iraq. Many were students or businessmen whose real role, authorities said, was to spy on exiled dissidents and feed intelligence information to Baghdad. German police Wednesday raided 50 Arab homes and arrested four Arabs, accusing them of plotting terrorist attacks.

In the United States, Bill Baker, an assistant Federal Bureau of Investigation director, said the FBI is reviewing the incidents overseas "for any links that might mean there's something moving our way. We haven't found that yet."

Thomas E. DuHadway, FBI agent in charge of the Washington field office, said agents will interview individuals believed to have terrorist connections, "but we don't have any contingency plans to round up groups of Iraqis" or radical Iraqi supporters.

U.S. counterterrorism experts said one of the FBI's principal targets for investigation is the Abu Nidal Organization (ANO), which the State Department has labeled "the most dangerous terrorist organization in existence." Vincent Cannistraro, former Central Intelligence Agency chief of counterterrorist operations, said the ANO has had a "rudimentary infrastructure" in this country for several years.

"These are secret assets," Cannistraro said. "They haven't been doing anything, but they could be called on to carry out or support a terrorist attack. They are the ones of prime concern."

FBI agents also have started interviewing an estimated 3,000 Iraqi visitors whose visas have expired, but Baker said that "we're not at all certain we'll find all of them or many of them."

Visitors legally in this country are not required to report their whereabouts to immigration authorities.

Justice Department officials said this week that since the August invasion of Kuwait, the FBI has foiled more than five schemes that could have led to terrorist acts. The individuals involved appeared to be lone "zealots," the officials said.

While the domestic threat is regarded as far less pressing than the danger to Americans overseas, security precautions were stepped up again yesterday at airports and other potential targets, including the Trans-Alaska pipeline and the nation's Strategic Petroleum Reserve in Louisiana and Texas.

The Federal Aviation Administration ordered airlines and airports to implement highest security, adding guards and limiting airport access. Some airlines went further, especially on international flights. A Pan American official said that "for the next few days," the airline will accept no cargo on flights from Europe to the United States.

In incidents abroad, demonstrators threw rocks at windows of the U.S. Consulate in Lahore, Pakistan. A Woolworth's store in Bonn was firebombed. Two molotov cocktails were tossed into a video shop in Milan popularly known as the "American Video Library." And the Italian news agency, ANSA, reported that a container of gasoline was set on fire near a Harvard University center in Florence.

"These incidents may well be gulf-related," a knowledgeable U.S. official said, "but it is not clear they were Iraqi-directed."

Staff writers Al Kamen, Thomas W. Lippman, Jim McGee and Don Phillips in Washington and Steve Coll in Islamabad, William Drozdiak in Paris, Marc Fisher in Bonn and Glenn Frankel in London contributed to this report.