Terror struck back against the United States and its friends on three continents yesterday, in an explosion of vengeful fury ignited by the American bombing of Libya.

Terrorists "executed" three kidnap victims in Lebanon, tried to blow up an El Al jetliner in London, and tossed firebombs at U.S. targets in Tunisia.

In city after city around the world, angry crowds swirled around U.S. embassies and screamed their hatred for "U.S.A. Aggressor."

Bomb-disposal squads scurried around European capitals in response to threats. American embassy staff members were being airlifted out of Sudan. In Libya itself, nervous Westerners looked desperately for ways out of the country.

And in Moscow, in a possibly ominous development, the Soviet Union summoned foreign ambassadors and formally asserted its right to free passage through the seas and air around Soviet-aligned Libya, a move that might signal Kremlin intentions to move naval vessels or other military equipment into the tense Mediterranean.

"We are prepared for an increase in terrorism," White House spokesman Larry Speakes said in Washington.

In Lebanon, retaliation for the U.S. raid was swift and bloody.

The bodies of three kidnap victims--identified as writer Alec Collett, 64, and teachers Leigh Douglas, 34, and Philip Padfield, 40, all Britons--were dumped on a highway, each shot once in the head.

A note found nearby claimed they were U.S. and British spies and had been "executed" by "Arab commando cells" in reprisal for the attack on Libya. Collett was kidnapped 13 months ago and Douglas and Padfield last month.

Arab anger has been directed at the British government because it allowed the United States to mount the air attack on Libya from a U.S. air base in Britain.

Soon after the bodies were found, a British television cameraman was abducted by gunmen in Beirut. Eighteen kidnapped foreigners, including six Americans, are still missing in Lebanon.

In Tunis, someone in a passing car hurled a firebomb at the compound housing U.S. Marine guards and other staff members of the U.S. Embassy in Tunisia. An embassy spokesman said an American-owned automobile caught fire but no one was hurt. Later, two young Tunisians on a motorbike set two more cars ablaze with firebombs near the compound, police said. One man was arrested.

To the south, in the Sudanese capital of Khartoum, the U.S. Embassy was preparing for an airlift evacuation of more than 200 embassy staff members and dependents.

Fifteen thousand protesters demonstrated Wednesday in Khartoum, burning American flags and chanting, "Down, Down U.S.A."

Since Tuesday's U.S. attack on Libya, the tight security at embassies and other U.S. installations worldwide has been toughened still further. Embassies are receiving about a dozen credible bomb threats a day, State Department officials said.