After a night of violent attacks by firebomb-hurling demonstrators, Tokyo's new international airport finally received its first airplane this morning.

But right up until the last minute, the 15,000 radical protesters who had laid siege to the heavily fortified airport continued their efforts to keep it from finally being put into operation.

As the first flight, a japan Airlines cargo plane from Los Angeles, neared Japan, protesters threw up a new challenge, launching dozens of kites and ballons into the air six miles south of the airport and directly in the path of the runaway.

The plane, however, was late. The ballons drifted away and police had time to cut down the kites, which had been staked to the ground, before the cargo jetliner landed safely.

A widespread power blackout also occured this morning in towns and villages near the airport at Narita, a suburb 40 miles from Tokyo, it did not affect the airport, which has its own power supply. Radical opponents had threatened earlier to attack the airport's power and water supplies.

The airport, center of a bitter controversy since it was planned 12 years ago, had been assaulted at several previous points during the night. But the 13,000 police massed there this week-end fought them off with teargas. ADD ONE JAPAN - F

In the heavieist fighting last night, about 20 police were hurt when a band of demonstrators, attempted to storm an airport gate by crashing into it with two burning trucks. Police pushed them back with tear gas and arrested about 50 persons.

In other encounters, a truck was burned and demonstrators hurled rocks at police who responded with water cannon.

There were several ne w attacks on aviation facilities located far away from the Narita airport. One group threw firebombs in an unsuccessful attempt to break through a gate at a radar station of the Tokyo Aviation Bureau about 30 miles east of the airport.

A radio beacon station that serves the present airport at Haneda, nearer to Tokyo, was also assaulted and some equipment was damaged by firebombs and lead pipes. The radar is used to assist planes landing at Haneda. The extent of the damage was not immediately known.

The protesters, most of them young radicals wearing helmets, staged marches on roadways surrounding the heavily fortified airport grounds.

In a series of public statements, the radicals and farmers who have fought for nearly 12 years to keep the airport from opening threatened to continue the assaults.

The pattern of attacks showed that the opponents had learned a great deal about the airport's vulnerable points and that the police will have many problems preventing damage that could shut down the facilities at both Narita and Haneda.

During a three-hour period Saturday morning, Haneda officials had to ground 114 domestic flights after members of one redical faction cut an underground microwave cable at two points. That knocked out Haneda's redar and shut off voice communications between teh control tower and flights landing and taking off.

The radicals inflicted that damage by locating and unsealing two man-hole covers to reach the cable. They masqueraded as utility company employes making repairs and set up official-looking wooden barricades around one manhole while they worked.

Later, police said they thought the radicals must have had assistance from someone inside the cable company in order to locate the two places where they cut the wires.

The Narita airport was officially declared open Saturday morning at a brief Shinto ceremony attended by 50 officials who gathered in a room of the heavily guarded terminal.

Kenji Fukunaga, the transport minister, offered the day's only humor when he used an old Japanese proverb to describe the long delays in opening the airport. The proverb says that a baby delivered after a long and painful birth will grow to be a healthy child, Fukunaga said.

It was in July, 1966, that the Japanese cabinet finally decided to construct the new international airport in an area northeast of Tokyo that was then open farmland.

Embittered at seeing their land taken, farmers filed lawsuits and began protesting. They were soon joined by thousands of young radicals many of whom had become protest veterans during the anit-American demonstrations of the 1960s.

The airport's first phase was completed in 1971 but it was remained unused. Plans were made and abandoned to open it twelve times. In the demonstrations and riots, five persons were killed, 2300 arrested, and at least 8,000 injured.

Many problems still lie ahead. Only one runway has been buile and 17 farmers are determinedly holding on to the land needed to build a second one.

There are repeated claims that the airport is not safe. Pilots have been quoted as warning that the area is swept by strong, gusty winds that make landings hazardons. Some authorities have noted that the approach and takeoff zone is narrow because aiirspace in the vicinity is largely taken by two military bases.