It was standing room only on the No. 1 train, like so many other weekday mornings, when Robert Rios started inching toward the metal doors under lower Manhattan yesterday.

The train was pulling into Chambers Street, and the first-year math student at Borough of Manhattan Community College wanted to beat the rush to the stairs.

"The doors opened and somebody said, 'There's a bomb in the subway,'" said Rios, 24, who lives in Ridgewood. "It was crazy. A bomb in the subway?"

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Actually, the hands of terror were mangling the heart of New York more than 100 stories above where Rios was standing.

Word filtered down to the subways that something terrible had happened. A subway passenger screamed. Others pushed and shoved. There was panic, then a mad rush for the exits.

Rios climbed up to the street. Bodies rained down from the clear, blue sky.

"People were jumping from the burning building," said Rios, still shaking nervously one hour after the first blast. "It was like war."

This would be like no other weekday morning in New York.

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Jake Birner, 24, a temporary worker, emerged from the Canal Street station on the A line to find a gaping hole in that once-unmistakable New York landmark the World Trade Center. A dark plume of smoke swirled toward the sky.

Birner just walked south, toward the flames. Other people pouring out of lower Manhattan subway stations stopped to stare.

"Then we see a plane slam into the other tower," Birner was saying. "There was a big, giant fireball. It was like science fiction. Traffic stopped and people were getting out of their cars. People were screaming and crying and shouting up to God, 'Why? Why?' Some people stopped and prayed."

The entire subway system, all 722 miles, was essentially shut down yesterday from 10:20 a.m. to shortly before noon, according to New York City Transit spokesman Al O'Leary. It was the first time the city's subways had stopped since the power blackout of July 1977.

There were scattered third-rail and signal power outages in the subway system about an hour after the first plane struck the World Trade Center. At 10:20 a.m., transit officials halted subway operations.

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"We were losing power in various places," O'Leary said. "It became clear there was some kind of threat. Rather than have people trapped underground the decision was made to discharge passengers and secure the trains."

The subway stations under lower Manhattan were eerily quiet shortly after noon. Soot filtered down to the stations near the World Trade Center, covering the floors, the phones, the MetroCard vending machines. Token booths were empty.

Under Chambers Street, at 12:20 p.m., a lone C train idled at the deserted station. The walkways leading to the PATH commuter trains were covered with up to 2 inches of soot. Occasionally, a confused soul would venture down to the subway station only to turn back.

In Herald Square and Times Square, hundreds of people could be seen standing on the streets outside train stations. Some sat at curbsides, talking. Some cried as they were comforted by strangers.

Most of the subway system, except for the trains under lower Manhattan, was running by 2:30 p.m.

O'Leary said there were a number of bomb threats throughout the day. None turned out to be real, he said.

Transit officials expected to get into the World Trade Center and Courtlandt Street stations last night to inspect their structural stability.

"Our big question is the condition of both stations," O'Leary said. "We're going to get in there as soon as we can."

Many New Yorkers who depend on the subways walked yesterday, crossing bridges into Brooklyn and Queens before catching buses home.

Abdula Jones, 44, who witnessed the terrorist assaults with co-workers from a building across from the World Trade Center, crossed the Manhattan Bridge by foot with hundreds of strangers. He rides the subways every day, he said, and had never seen New Yorkers so united.

"Once again, tragedy has brought us together," he said. "Usually New Yorkers are tense and hostile during their commute. We put all that aside for a moment. People were holding onto each other and comforting each other. I walked across the bridge with a man I met on Canal Street. A stranger. We talked like we knew each other for years."