It was just before 2 a.m. on Aug. 21, 1968, when the high-pitched sound of aircraft landing one after another at Prague's Ruzyne airport pierced the stillness of the summer night.

That repeated sound was the first sign to most people in Prague that the Soviet Union was moving to crush the "Prague Spring," the series of reforms spearheaded by Czechoslovakia's new Communist Party chief, Alexander Dubcek.The noise of the aircraft shattered the illusion that the Soviets would let Dubcek go ahead with his plans.

I flicked on a small portable radio and just caught the words ". . . an important announcement from the Central Committee . . ." before the transmission went dead.

Then I charged down a couple of flights of stairs and heard the rest of the message in the Alcron Hotel's garage, where the public address system was hooked into Prague Radio - a common setup in Czechoslovak factories and public places.

There was no mistaking the message:

"Yesterday, the 20th of August 1968, around 2300 hours (11 p.m.), armies of the Soviet Union, the Polish Peoples' Republic, the German Democratic Republic, the Hungarian Peoples' Republic and the Bulgarian Peoples' Republic crossed the state borders of the Czechoslovak Socialist Republic."

I darted for the hotel's telex machine and punched out the first bulletin on the invasion: "Radio Prague announced tonight that Soviet troops have crossed the Czechoslovak borders and asked Czechoslovak citizens not to take action against them . . ."

Outside, I found dozens of cars driving up and down, flashing lights and honking horns to alert the sleeping population. The drivers had heard the radio and the planes.

In the Alcron's lobby there was pandemonium as tourists fired questions at the night staff. A waiter angrily asked news correspondents: "Why didn't you tell us the Russians were coming?"

I saw my first Russian soldiers at midmorning from the balcony of the U.S. Embassy. An armored half-track with an open top appeared on the steep street in front of the embassy. The half-dozen helmeted soldiers nervously clutched their rifles and stared at the American flag.

A skinny civilian in a dark suit was fidgeting with a tourist map of Prague, trying to direct the driver. The vehicle, obviously lost, made a U-turn and headed back down the hill, its treads sliding on the street.

By midmorning the old town section of Prague was filled with people, many carrying briefcases, who had been on their way to work. They glared at the soldiers, yelling angrily: "Why did you come?" "What do you want here? Go home!"

Small groups gathered around transistor radios, listening to broadcasts that announced the arrests of Dubcek and his allies.

Tanks guarded the approaches to the Vltava River bridges. Soldiers stopped cars and searched for pro-Dubcek newspapers and leaflets.

Prague's printing presses had been busy. People in trucks full of newspapers threw bundles to passers-by, who grabbed them up as fast as they could reach them.

Rude Pravo, the official organ of the Communist Party, published a one-page edition, splashing prominently the text of the Central Committee's declaration announcing the unwanted invasion.

By midafternoon the great square at the center of Prague looked like a spent battleground. Debris was everywhere. Tanks were parked on all sides, their cannons pointing into the air. Young people marched up and down, waving Czechoslovak flags and yelling "Dubcek."

The square was filled with the smell of burned gasoline. Blue smoke from tank engines hung in the air. The tank crews looked weary as young men and women yelled the now familiar question: "What do you want here? Go home!"

The crews were tense but controlled while officers strolled among the young Czechoslovaks trying to convince them that the Soviets had come in answer to a call for help.

The action was thickest around the statue of King Wenceslas, draped with young people arguing with a Soviet political officer.

Jan, an engineer I had met earlier, popped out of the crowd and showed me a shell casing. He said there had been fighting around Radio Prague, just beyond the square.

We had a look at two burned out city buses and a burned out Soviet tank and truck. We were told that some young Czechs had set the truck on fire and that it carried ammunition which exploded across the crowded area.

My acquaintance said the tank driver panicked and opened fire on two young men on the back of a passing truck. Others still at the scene said four people had died and many had been injured. Jan said he would take the shell casings home as a souvenir.

There were other shooting incidents throughout the country; about 100 people were reported to have been killed during the invasion.

The parade of people on Wenceslas Square went on throughout the evening. The youngsters were angry. Elsewhere, there were tears.

"I remember when the Germans came and now this!"

The Soviets had imposed a 10 p.m. curfew - which the underground radio now broadcasting non-stop took pains to announce. But at 8 p.m. the Soviets started clearing the streets, firing into the air. It seems they were going by Moscow time.

Hundreds of foreign tourists added to the confusion. An international geological congress had been under way, and a multiple sclerosis association meeting had also been scheduled. Its star participant was to have been Shirley Temple Black, who was staying at the Alcron.

My wife, Louise, rounded up the former child star for an interview. I now had to use the telex at the U.S. Embassy because the Alcron, fearing reprisals, had pulled the plug on its machine. Louise left a few days later with our 6-month-old daughter, Anna, Shirley Temple and dozens of other foreigners in an embassy-organized convoy.

The Soviets had counted on Dubcek's opponents to gain political control with the help of the military intervention. But the hardliners failed to muster any popular support.

For four days the protests and demonstrations kept up, backed by pro-Dubcek radio and television broadcasts which the Soviets were unable to shut down. Almost every man, woman and child in Czechoslovakia had rallied behind the reform leadership. The country's 72-year-old president, Ludvik Svoboda, refused to endorse a pro-Soviet puppet regime.

The support for Dubcek forced the Soviets to negotiate with their prisoners, and Svoboda, a war hero who had commanded the Czechoslovak brigade that fought with the Red Army in World War II, was flown to Moscow to join the talks.

The Czechoslovaks had to sign an agreement for the "temporary" stationing of Soviet forces. Dubcek was reinstated, but the reforms were dead. With Soviet troops in the country, the pro-Soviet faction gained control. Dubcek was deposed a few months later when crowds rioted to celebrate a hockey victory over the Soviet Union.

The Prague Spring was over.