It was dusk and the sky glowed red from the fire blazing at the U.S. Embassy seven miles to the north, when Wendy Koritki spotted a mob of Pakistani youths pouring over the fence of the International School of Islamabad.

"They're here. They're here. They're coming over the fence," screamed the 13-year-old ninth grader as she raced back into the school building.

Thus began the terror yesterday for about 60 American children who were caught at school by rampaging mobs, which burned the American Embassy here, the American libraries in Rawalpindi and in Lahore and businesses with "American" in their names here and in Rawalpindi.

While the focus was on the 90 people trapped for seven hours in the burning embassy building, the Americans threatened elsewhere faced hours of terror until Pakistan's Army-run martial law government finally sent in troops to control the mobs.

Thomas F. Putscher, a 32-year-old Agency for International Development auditor, was taken as a hostage to Quaid-i-Azam University for 7 1/2 hours after being beaten by the mob.

One woman was reported to have been so scared after getting out of the burning embassy that she hid in the bushes instead of seeking sanctuary with the others in the British Embassy. Another woman spent seven hours huddled with her children in a quiet corner of the embassy compound hoping the mob would not spot her.

Perhaps the most extreme terrorism was felt by the children at the school, who knew the embassy was surrounded and could see its flames lighting the sky.

"We were worried about our parents," said Devin Haggerty, 17.

Most of the other foreign children already had been taken home by late yesterday afternoon, and the Americans had been divided by neighborhoods so diplomats from other countries could take them to safety in car pools.

It was then that the Pakistani mob invaded. The children were moved quickly into a small music room.

"We heard windows breaking," Haggerty, a senior, said. "We heard doors being pounded on 10 or 15 yards from where we were."

What they did not see was the strong defense put on by Pakistani employes. They were led by a retired military officer, Ismil Khan, a former Pakistan chief of protocol who was once the military attache in Washington. He had come to the school when he saw the mob climbing the fence.

He quickly grabbed baseball bats and brooms and organized school employes to begin beating on the mob, which fell back as police and troops arrived.

About 15 students had taken refuge in the darkened auditorium. They did not know it was safe to come out, and when a Pakistani employe asked if anyone was there they were too afraid to answer. Their terror lasted about an hour longer.

Meanwhile at the embassy, Putscher was among 16 persons trapped while eating lunch in the American Club on the embassy grounds.They hid for hours in the dining room until the building started blazing. When they ran out, said Putscher they were beaten with bricks, sticks and fists while police and army troops stood by and did nothing.

"They didn't do much to prevent me from being dragged away. I am just glad to be alive," said Putscher, who had a gash on his nose, a sore back, a slight concussion and possible internal injuries.

He said he was dragged off to the university campus and held as a hostage from 4:40 p.m. to midnight yesterday. The other 15 persons were finally put in a police truck and taken for the night to the military cantonment in nearby Rawalpindi for their own safety.

Marine Sgt. Mike Sirois, 27, of Harvey, Ill., was treated at the British Embassy today for burns on his forearms picked up during the battle for the embassy. He was not sure how he got them, but he said they probably were caused by the tear gas canisters he was tossing at the invading mob.