Gen. Alexander Haig, supreme commander of allied forces in Europe, narrowly escaped an apparent assassination attempt early today when a remote-controlled bomb was detonated under a small bridge in Belgium as his car drove over.

Another car carrying two Belgian and one American security men just behind the general's sedan was demolished in the blast that tore a hole 10 feet wide and 3 feet deep in the bridge.

Haig, his driver and adjutant were unhurt as the blast, which threw their Mercedes 600 sedan into the air, came just after their car passed over the bomb.

At a press conference later at his headquarters in Mons, Belgium, Haig told reporters he was very happy to be alive and that had the detonation come "a fraction of a second earlier" he would have been killed.

The second car was destroyed by boulders and debris thrown up by the blast but the security men inside received only minor injuries.

The incident, the first of its kind that officials of the North Atlantic Treaty Organization can remember, came just five days before the 54-year-old, four-star general is scheduled to retire after 35 years of military service and 4 1/2 years as the top NATO commander.

Through Haig has been noncommital thus far on his plans, he has been mentioned as a possible Republican presidential or vice presidential candidate.

Asked if he thought the attack might be linked to his being a presidential hopeful, Haig joked that "I don't think I have quite enough support in the United States at the moment to justify such drastic action."

A NATO spokesman, Sjouke de Jong, said in a telephone interview that no suspects were in custody. Belgian state police are handling the investigation and, while there is some idea of how the attack was carried out, there reportedly is no indication of who was responsible, nor has any group claimed responsibility. There is only speculation about the motive.

One theory centers around the recent announcement in Washington by out-going Army Chief of Staff Gen. Bernard Rogers, who will replace Haig here as NATO chief, that the army plans on setting up a 110,000 man strike force to respond to trouble spots, such as the Persian Gulf or Middle East, in a crisis.

Haig told the news conference that there had been terrorist threats against him in the past from various groups and that about a year ago he had been made a clear target for one specific group, which he refused to identify publicly or tie to today's attack.

He called the attack a "strong reminder" that there are still forces of extreme right and left who want to change things with violence and that the incident was "an unhappy manifestation of the confused times in which we live."

Haig also called the attack "very professional." De Jong, who saw the blast area, estimated the charge involved between 100 and 300 pounds of TNT or plastic explosive.

A wire cable led about 150-200 yards away from the bridge, indicating the device had been set off by remote control. Police also reportedly found a walkie-talkie at the scene and there were reports that persons in the area had heard the sound of motorcycles, suggesting that the two-car convoy was under observation by members of an assassination team who were in touch with whoever was manning the detonator.

A NATO spokesman said that Haig took various routes to work. The fact that the assassins selected the one they did indicated that they had been observing the general's movements for some time. The attack took place at about 8:30 a.m. on a secondary road in the town of Obourg about half-way between Haig's headquarters in Mons and his home, about seven miles east. The bridge which spanned a culvert, was 50 feet long.

DeJong said that from the style of the attack it undoubtedly took preparation and patience, meaning it probably required more than one night to set up and had been in place for a while as the attackers waited for Haig to take that route. CAPTION: Picture 1, ALEXANDER HAIG . . . bomb damages car; Picture 2, Two Belgian explosive experts remove wire used to detonate the device employed against NATO cars. AP