A police officer patrols a platform as a subway trains arrive and leave a station in the Brooklyn borough of New York

New York City's subway system is operating normally, a day after authorities said they had received a credible terrorist threat. Millions of New Yorkers went to work as usual, under the watchful eye of police.

A section of one of New York's busiest train stations was sealed off Friday morning, as authorities investigated suspicious items. Police at Penn Station in midtown Manhattan donned protective gear to remove a green bottle, which Police Commissioner Raymond Kelly said appeared to have been placed by a prankster.

"In that bottle was a dye and an acid," said Raymond Kelly. "It looks like a prank. I would describe it as a drano-like [drain cleaner] substance. Our police, emergency services officers went in, made an initial determination and brought it to the Department of Environmental Protection."

New York City Police check subway cars at Columbus Circle

That incident underscores the increased security precautions in effect throughout the 1,000-kilometer-long city subway system. But for most of the 4.5 million commuters who use the New York subways each weekday, Friday's travel was no different than usual.

Trains pulling into the busy Grand Central Station were packed, as on any other day. Manhattan native and backpacker Larry Goodman said the heightened security had not affected him at all.

"As a matter of fact, I've taken five separate subway trips since yesterday, and I've had this backpack with me at all times, and nobody's stopped me once," said Larry Goodman.

Mayor Michael Bloomberg talks to a subway passenger, Friday morning

New York Mayor Michael Bloomberg rode the subway to his office Friday morning, as he usually does. The night before, while warning New Yorkers that a specific threat had been received, the mayor also made clear there was no need to panic.

"I believe we have an obligation to share information with the public, as long as it doesn't jeopardize their safety, and they can make their own decisions," said Michael Bloomberg. "I believe that they should live their lives like they always do."

Most commuters Friday seemed to be taking the mayor's advice. Carlita Rosario, an immigrant from the Dominican Republic, says she has ridden the subway regularly since she arrived in New York 12 years ago, and feels safe.

"I'm not worried, because this country has a lot of security," said Carlita Rosario. "No. I'm not afraid. I'm feeling sure."

Security at Grand Central Station was not noticeably heavier than on any other day.

National Guard Sergeant Andres Santana stood in the bustling station lobby, manning a post established after the September 11, 2001, terrorist attacks. Wearing a heavy bullet-proof vest in the hot, stuffy hall, he said New Yorkers appear to be taking the threat in stride.

"I guess people figure they've got to live their life normal every day, till something happens," said Andres Santana. "You can't be every day worrying about what's going to happen tomorrow."

Hassan Islam, an immigrant from Bangladesh, operates a news stand on the subway platform at Grand Central Station. He was philosophical about the threat of terrorism, and said he intends to carry on with his life.

"I'm worried, like, everybody want to save their life," said Hassan Islam. "I like to save my life. Working hard, making money.  I really like this country, so…"

Conference Gbaje, an immigrant from Nigeria, said he uses the subway every day to get to his job in midtown Manhattan. When asked whether he feared for his life, he pointed to the tragic case of elderly bus passengers killed in a freak accident, while fleeing the recent hurricane in Texas.

"Do you remember what happened in Hurricane Rita, when people were taken away with the bus, and they died? What if they did not go with the bus, they could have been alive today. So, you don't know. Anything can happen, anytime," said Conference Gbaje. "Everything's all right in New York."

In Washington, officials played down the seriousness of the threat. White House spokesman Scott Mc Clellan said it was "of doubtful credibility." Asked about the reaction in Washington, Mayor Bloomberg defended his decision to warn the public.

We've gotta [we have to] worry about doing what's right for the people of this city," said mayor Bloomberg. "If, given the same circumstances again, I would make exactly the same decision."

New York's security alert level Friday remained at orange - the second highest level, the same as it has been since the September 11, 2001, attacks.