NEW YORK -- A scene of unbelievable horror unfolded before the eyes of thousands in downtown New York this morning.

A passenger jet hit the One World Trade Center, the northern tower of the landmark 110-story World Trade Center at about 8:50 a.m. ET, followed by a second jet that hit the tower's twin building about 10 minutes later.

Two World Trade Center, the southern tower, collapsed on itself in a deafening explosion, sending endless plumes of black smoke pouring from the tallest buildings in New York City.

One World Trade Center, the northern tower, collapsed several minutes later, engulfing the southern tip of the island of Manhattan in smoke, ash and debris.

Injured people were spread in a wide area, including burn victims and survivors who lay on the sidewalks after collapsing from exhaustion.

"It's mass pandemonium," ABCNEWS.com reporter Corey Goldman said moments after the second tower collapsed. A horrible burning smell filled the air as people ran from the area. Officials had attempted to evacuate the area around the towers after the planes hit. Thousands of people were gathered in the area.

"Everyone was running," Goldman said.

After the two buildings collapsed, the scene became eerily quiet, witnesses said. People came together in the streets, hugging, crying, and forming small prayer circles, asking God to spare their friends and loved ones.

A Normal Morning Torn in Two
When the first plane hit the northernmost building, some inside thought it was an earthquake. Others suspected a bomb.

"It was a solid explosive shock," said Richard Pico, a doctor who was on the 19th floor of One WTC, the northern building. "People walked out of their offices. People were screaming loudly, not knowing what to do."

"I thought it was a bomb," said one witness. "The ceiling started to collapse and then we evacuated."

The man was on the 81st floor of the no when the plane hit, and he and his co-workers ran down all 81 flights of stairs to escape the disaster. They were soaked with sweat and water from the sprinklers when they emerged from the building.

Speaking in a slow, stunned tone, Tani Hironaka, who had been on the 80th floor of One WTC, said: "I thought it was an earthquake at first."

Horror Outside the Building
People outside the buildings said they could hardly believe their eyes as they saw the planes hit.

"I was looking at the smoke and saw a plane heading low over lower Manhattan," aid 33-year-old Jennifer Tammi, a doctoral student who was taking the subway to classes at Columbia University. She was riding the F train, which travels above ground along part of its route.

"It banked and headed straight for the middle of the other tower. It looked like it flew right through the building," she said.

"Guys were screaming. I heard the noise," said Theresa Kent, who works in an office building directly across from the World Trade Center buildings. "I was in my office and I heard a guy screaming so I ran out and then everybody was saying, 'Oh my God' and there were tears. They were crying."

Terrifying Evacuation
After the explosion in One WTC, security ordered Pico and his co-workers to evacuate. With smoke already seeping into stairwells packed with evacuees, they ran down and out of the building. Victims assembled by the docks on the western side of Manhattan.

It was unclear whether the area was protected when the two buildings collapsed.

As people began flowing out of the building, the scene became increasingly chaotic, said Katherine Fegan, who works at Salomon Smith Barney.

"Many people were crushed people were going crazy, crying, and everyone was running," she said.

"Sometimes it was calm and orderly, other times it was complete bedlam," said Tom Grassi, who was in the North Tower.

Some witnesses said they saw people jumping from windows in the World Trade Center. Others had run to the roof, hoping to be lifted to safety.

Outside, shocked survivors and witnesses gathered in the streets, standing in an inch and a half of ash and dust and a torrent of falling debris and papers. Cell phones and many pay telephones were inoperable in the wake of the disaster. Witnesses said they saw rescue workers rushing into the buildings to pull people out.