NEW YORK — The worst power failure in history blacked out and crippled the mighty cities of the Northeast Tuesday night, trapping hundreds of thousands in stalled subways and elevators and affecting up to 30 million persons.

President Johnson ordered the Federal Power Commission, with the help of the FBI, to launch a sweeping investigation of the blackout which enveloped New York, Boston and countless cities and hamlets in half a dozen states.

Mr. Johnson was advised, however, that utility experts were “pretty well agreed” that no sabotage was involved, although no one seemed sure where or how the breakdown occurred.

The failure hit during the rush hour, at about 5:30 p.m., and made a mess of transportation and communications. Even though the lights began flashing on in Boston and most other areas hours afterward — including Ottawa and Toronto in Canada — glittering New York was still a dark, powerless city as the clock crept into a new day.

At one time, more than 850,000 persons were trapped in stalled cars in New York’s subway system.

Looking up Broadway’s “Great White Way,” it appeared as if thousands of fireflies were lighting up the area as New Yorkers trudged along dark streets carrying flashlights.

An airline pilot coming in for a landing at Boston’s Logan International Airport at 5:21 p.m. saw “a startling sight. There below is a brightly lighted city and suddenly it plunges into darkness. You just don’t know what to think.”

Like the pilot, thousands upon thousands of Easterners were caught in mid-stride.

Elevators stopped between floors. Subway trains slid to a halt in the miles of labyrinths under New York and Boston. Movie screens went black. Airliners sought out landing fields outside the blacked out zone.

Off-duty policemen were called back to work. National Guardsmen in some areas were alerted in case of looting, and convicts at the Massachusetts state prison at Walpole took advantage of the excitement to throw a riot which was quickly quelled.

The great luminous cities looked as if they had been struck by some awesome tragedy. But reports indicated most people took it all calmly. Restaurants did a thriving business by candlelight.

As late as three hours after the power failure hit, the New York Transit Authority estimated 300,000 passengers were still stranded in subway tunnels.