When night falls on Liberty City, power changes hands. Dade County police pull back from their heavy daytime patrols, and snarly knots of teen-agers spill out of the hot and crowded James E. Scott public housing project and take charge.

For the past two nights, youths with revolvers and shotguns have milled along 22nd Avenue, the wide main street of this bleak ghetto. They throw rocks and bottles at white motorists and city buses, push heavy trash-truck bins in the path of freight trains and sporadically fire volleys at heavily armed caravans of police who make hourly sweeps looking for persons stranded, injured or worse.

County police have been ordered to fire only in self-defense and to avoid aggressive actions. This has created a nighttime zone in which robberies, burglaries and vandalism are tolerated. Police officers are outraged.

Older black and white residents of the Miami area have been flooding the county building with expressions of fear, and today county manager Merrett Stierheim announced a 9 p.m. curfew in the area.

Black leaders like Urban League president T. Willard Fair denounced the violence and continuing disorders.

Those who are sincerely concerned about improving the quality of life," said Fair, his eyes flashing, "I'm going to try and help. Those who just want to break the law, I want to put in jail."

Sgt. Hugh P. Peebles, president of the Patrolmen's Benevolent Association, the local police union, said the nightly pullback of police has "planted in the minds of rioters that they have won a victory."

Peebles said police are threatening to walk out. "Those kids are better armed than we are and they have no restraints. They don't have to read the rights to police. It's a big game to them."

Fair and some black officers who patrol Liberty City expressed deep anxiety that the continued disorders would prompt strong police action and a potential bloodbath.

But there was little indication that their worry was having much effect on the youth of the Scott project and the other black teen-age males who come from other neighborhoods in Miami at night to the scene of the action.

Late in the afternoon, they gather in bands of five to 25, hanging around parking lots, spilling out of bars and grocery stores. Most are between 15 and 19 years old and they are resentful about the cycle of poverty in which they find themselves, so close to Miami's beaches and luxurious hotels. Many sneer at older blacks, even 30-year-old men in the project, who have attempted to counsel calm and nonviolence.

"It's 1980: they've dealing with a new breed," said William Mcgee, 24, who often comes to the projects at night to watch the scene. "The new breed is impatient. They want action. When they want action, they want it now.

"They got all these beautiful sights here looking at the ocean. I'm tired of all these crackers [whites] looking. I want to see the ocean my damned self.

"I guess you can say black people tried it all kinds of ways, tried it the political way, tried it the talking way. Now if this doesn't work, I don't know what we'll try next. But it's going to happen. These white people are going to have to give up some equality."

As McGee stood up with a group of about half-dozen other youths and talked Wednesday night, intermittent gunfire crackled from two blocks down the street. When the guns went off, the youths would cheer, saying they now had the upper hand with police, who they regard as pushy, arrogant and brutal.

"Any fool that stays home and says I ain't got nothing to do with it, they're fools," another black youth said.

A boy of about 10 on a bicycle pedaled back and forth from the perimeter to the milling armed youth who were firing. He was jubilant as he talked about what was happening.

"It was wall-to-wall brothers on each side," he reported one time. "And anything that wasn't black was just a target."

Both police and project youths estimated that 200 teen-agers milled about in the streets Wednesday night. When officers attempted to make a sweep through the area shortly before midnight, the teen-agers bragged that they had set up an ambush, with armed youths in bushes on both sides of the boulevard. Police said they could see no one until they drove into the ambush and suddenly were fired upon from both sides.

Officer Richard Kolodgy, 25, was driving one of the 15 to 20 patrol cars in the convoy that was ambushed as police used tear gas in an attmept to clear crowds from the main street of Liberty City.

"Just as we were lobbing our [tear gas] canisters, the shots started. We couldn't see who was shooting. Only the flashes. We returned the fire instinctively," said Kolodgy, a three-year member of a force that has been assigned to Liberty City the past nine months.

No one was hurt in the exchange, during which dozens of shots rang out in both directions. One bullet hit a door of Kolodgy's cruiser.

Kolodgy said he and his partner fired several times "in the general direction" of the flashes, which came from the darkened pathways that separate the two-story buildings of the housing project.

Because of the restraints on aggressive actions ordered by police officials, Kolodgy said, "My captain wasn't too pleased with the maneuver, but when you're being fired upon, you react."

Officer Michael Kernbach, 25, said "personally safety became our number one concern. It's a strange feeling . . . people deliberately attempting to hurt us, kill us, and we're just supposed to take it."

Kernback said, "You never heard so much bitching" over the police radio. The patrolmen were "a riding target" being directed by superiors who Kernbach called "a bunch of guys from downtown who have no conception at all" of what it was like in the combat area.

Kernbach said many officers came "very, very close to walking off" when they were ordered to stand back while looters loaded "dozens of refrigerators" onto a truck. "They came back for a second load," Kernbach said incredulously.

The appliances were taken from the community health center of the housing project shortly before it was set on fire.

In another incident, Kernbach said a crowd threw stones at a city bus and tried to overturn it. When the police move toward the bus, Kernbach said in amazement "our lieutenant said, 'I don't want a confrontation because today's my birthday.' That's when we got out. We realized that if we listened to him we were going to get hurt. Most of us drove off, parked and went to sleep.

Early this morning, Officer Charles Mussoline was among police assigned "to retake the railroad crossing," the train tracks blocked by youths along the north boundary of the troubled zone. Mussoline said police were escorting a Florida East Coast freight across the 22nd Avenue crossing when "two TV [station] helicopters flew over and woke everybody up. In a few minutes there was a crowd out there, cheering and taunting, saying. 'We're going to get you tonight.'"